The information in this prospectus is not complete and may be changed. We may not sell these securities until the Securities and Exchange Commission declares our registration statement effective. This preliminary prospectus is not an offer to sell these securities and we are not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

~~We are distributing~~Up to ~~holders~~26,785,714 Units (each Unit contains one Share of ~~our common stock~~Common Stock and ~~to holders of certain of our outstanding warrants who are entitled to participate in this offering, at no charge, non-transferable subscription rights~~one Warrant to purchase ~~units. Each unit, which we refer to as a Unit, consists of~~0.025 share of ~~common stock and tradable warrant representing the right~~Series B Convertible Preferred Stock

Up to 26,785,714 Pre-Funded Units (each Pre-Funded Unit contains one Pre-Funded Warrant to purchase one Share of Common Stock and one Warrant to purchase 0.025 share of ~~common stock, which we refer~~Series B Convertible Preferred Stock)

(Up to as669,643 Shares of Series B Convertible Preferred Stock Underlying the ~~Warrants. We refer to the offering that is the subject of this prospectus as the Rights Offering.~~Warrants) and

In(Up to 26,785,714 Shares of Common Stock Underlying the ~~Rights Offering, you will receive one subscription right for~~Pre-Funded Warrants)

We are offering up to 26,785,714 Units (“Units,” each ~~share~~Unit consisting of common stock or each share of common stock underlying our participating warrants owned at 5:00 PM Eastern Time, on , 2016, the record date of the Rights Offering, or the Record Date. The common stock and the Warrants comprising the Units will separate upon the effectiveness of the exercise of the rights and will be issued as separate securities, and the Units will not trade as a separate security. The subscription rights will not be tradable.

Each subscription right will entitle you to purchase one Unit, which we refer to as the Basic Subscription Right, at a subscription price per Unit of $ , which we refer to as the Subscription Price. The Warrants entitle the holder to purchase one share of ~~common stock at~~Common Stock, par value $0.01 per share (“Common Stock”) and one warrant (the “Warrant”) to purchase 0.025 share of our Series B Convertible Preferred Stock, par value $0.01 per share (“Series B Preferred Stock”)). Each Warrant contained in a Unit has an exercise price of $ per share of Common Stock. The Warrants contained in the Units will be exercisable immediately and will expire on the 18-month anniversary of the date (the “Charter Amendment Date”) on which we publicly announce through the filing of a Current Report on Form 8-K that the amendment to our certificate of incorporation to sufficiently increase our authorized shares of Common Stock to cover the conversion of all outstanding shares of Series B Preferred Stock into Common Stock has been filed with the Secretary of State of the State of Delaware.

We are also offering the shares of Series B Preferred Stock that are issuable from time to time upon exercise of the Warrants contained in the Units. We do not currently have a sufficient number of authorized shares of Common Stock to cover the shares issuable upon the conversion of Series B Preferred Stock. As a result, before any shares of Series B Preferred Stock can become convertible, we need to receive stockholder approval of an amendment (the “Charter Amendment”) to our Tenth Amended and Restated Certificate of Incorporation, as amended, to sufficiently increase our authorized shares of Common Stock to cover the conversion of all outstanding shares of Series B Preferred Stock into Common Stock. We have agreed in the underwriting agreement for this offering to use our reasonable efforts to obtain such approval within 45 days from the date of ~~issuance through its expiration on . If~~this prospectus, and we intend to seek such approval at a special meeting of stockholders or at our 2018 annual meeting of stockholders. We cannot assure you ~~exercise your Basic Subscription Rights in full, and other shareholders or participating warrant holders~~that we will be able to obtain requisite stockholder approval of the Charter Amendment. The Series B Preferred Stock is not convertible until the next business day after the Charter Amendment Date starting at which time each 0.025 share of the Series B Preferred Stock will be convertible into one share of Common Stock. In the event our stockholders do not ~~you~~approve the Charter Amendment, the Series B Preferred Stock will not be convertible into Common Stock and the value of the Warrants and the Series B Preferred Stock may be negatively affected.

We are also offering to each purchaser whose purchase of Units in this offering would otherwise result in the purchaser, together with its affiliates and certain related parties, beneficially owning more than 4.99% of our outstanding Common Stock immediately following the consummation of this offering, the opportunity to purchase, if the purchaser so chooses, pre-funded units (“Pre-Funded Units,” each Pre-Funded Unit consisting of one Pre-Funded Warrant (“Pre-Funded Warrant”) to purchase one share of Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock) in lieu of Units that would otherwise result in the purchaser’s beneficial ownership exceeding 4.99% of our outstanding Common Stock (or at the election of the purchaser, 9.99%). Each Pre-Funded Warrant contained in a Pre-Funded Unit will be ~~entitled~~exercisable into one share of Common Stock. The purchase price of each Pre-Funded Unit will equal the price per Unit being sold to the public in this offering minus $0.01, and the exercise price of each Pre-Funded Warrant included in the Pre-Funded Unit will be $0.01 per share of Common Stock. This offering also relates to the shares of Common Stock issuable upon exercise of any Pre-Funded Warrants contained in the Pre-Funded Units sold in this offering. Each Warrant contained in a Pre-Funded Unit has an ~~over-subscription privilege to purchase a portion~~exercise price of $ per 0.025 share of Series B Preferred Stock. The Warrants contained in the Pre-Funded Units will be exercisable immediately and will expire on the 18-month anniversary of the ~~unsubscribed Units at~~Charter Amendment Date. We are also offering the ~~Subscription Price, subject~~shares of Series B Preferred Stock that are issuable from time to ~~proration, which we refer to as~~time upon exercise of the ~~Over-Subscription Privilege. Each subscription right consists of a Basic Subscription Right and an Over-Subscription Privilege, which we refer to as the Subscription Right.~~

The Subscription Rights will expire if they are not exercised by 5:00 PM Eastern Time, on , 2016. We may extend the Rights Offering for additional periods in our sole discretion. Once made, all exercises of Subscription Rights are irrevocable.

~~The Rights Offering is being conducted on a best-efforts basis. There is no minimum amount of proceeds necessary in order for us to close the Rights Offering.~~

~~We have engaged Maxim Group LLC to act as dealer-manager~~Warrants contained in the ~~Rights Offering.~~Pre-Funded Units.

~~Investing in our securities involves a high degree of risk. See the section entitled "~~~~Risk Factors~~~~" beginning on page 19 of this prospectus. You should carefully consider these risk factors, as well as the information contained in this prospectus, before you invest.~~

Wells Fargo Bank, N.A. will serve as the Subscription Agent for the Rights Offering. The Subscription Agent will hold the funds we receive from subscribers until we complete, abandon or terminate the Rights Offering. If you want to participate in this Rights Offering and you are the record holder of your shares or participating warrants, we recommend that you submit your subscription documents to the Subscription Agent well before the deadline. If you want to participate in this Rights Offering and you hold shares through your broker, dealer, bank, or other nominee, you should promptly contact your broker, dealer, bank, or other nominee and submit your subscription documents in accordance with the instructions and within the time period provided by your broker, dealer, bank, or other nominee. For a more detailed discussion, see "The Rights Offering—The Subscription Rights."

Our board of directors reserves the right to terminate the Rights Offering for any reason any time before the completion of the Rights Offering. If we terminate the Rights Offering, all subscription payments received will be returned as soon as practicable, without interest or penalty.

~~Our common stock~~Common Stock is listed on the ~~NASDAQ~~Nasdaq Capital Market ~~or NASDAQ,~~ under the symbol ~~"ONTX."~~“ONTX.” On ~~June 1, 2016,~~April 20, 2018, the last reported sale price of ~~our common stock~~Common Stock on the Nasdaq Capital Market was ~~$4.54.~~$0.56 per share. The actual offering price per Unit and Pre-Funded Unit, and the exercise price of the Warrants, as applicable, will be determined by negotiation between us and the underwriter at the time of pricing, and may be at a discount to the current market price. We do not intend to apply ~~to list~~for listing of the Pre-Funded Warrants, the Warrants or Series B Preferred Stock on ~~NASDAQ following their issuance under~~any securities exchange or other nationally recognized trading system. There is no established public trading market for the ~~symbol "ONTXW." The Subscription Rights~~Pre-Funded Warrants, the Warrants or Series B Preferred Stock, and we do not expect a market to develop.

For each Pre-Funded Unit we sell, the number of Units we are ~~non-transferrable~~offering will be decreased on a one-for-one basis. Units and the Pre-Funded Units will not be ~~listed for trading~~issued or certificated. The shares of Common Stock or Pre-Funded Warrants, as the case may be, and the Warrants can only be purchased together in this offering but the securities contained in the Units or Pre-Funded Units will be issued separately.

One or more of our directors have indicated interests in purchasing approximately 10% of the Units to be sold in this offering at the public offering price and on ~~NASDAQ~~the same terms as the other purchasers in this offering. However, because indications of interest are not binding agreements or commitments to purchase, the underwriter could determine to sell more, fewer or no Units to our director(s) in this offering, or our director(s) could determine to purchase more, fewer or no Units in this offering.

You should rely only on the information contained herein or incorporated by reference in this prospectus. We have not authorized any other ~~stock exchange or market. You are urged~~person to ~~obtain a current price quote for~~provide you with different information.

Investing in our ~~common stock before exercising your Subscription Rights.~~securities involves risks. See “Risk Factors” beginning on page 11 of this prospectus and in the documents incorporated by reference into this prospectus.

~~Our board of directors is making no recommendation regarding your exercise of the Subscription Rights. You should carefully consider whether~~Pre-Funded Warrants being offered pursuant to ~~exercise your Subscription Rights before the expiration date. You may not revoke or revise any exercises of Subscription Rights once made unless we terminate the Rights Offering.~~this prospectus.

We have granted the underwriter the option to purchase up to 4,017,857 additional shares of Common Stock at a purchase price of $ per share and/or Warrants to purchase up to an aggregate of 100,446 shares of Series B Preferred Stock at a purchase price of $0.01 per Warrant with an exercise price of $ per 0.025 share of Series B Preferred Stock, less the underwriting discounts and commissions. The underwriter may exercise its option at any time and from time to time within 30 days from the date of this prospectus. If the underwriter exercises the option in full, the total underwriting discounts and commissions payable by us will be $ , and the total proceeds to us, before expenses, will be $ .

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The underwriter expects to deliver the securities to purchasers on or about , 2018.

Table of Contents

TABLE OF CONTENTS


			Page
~~QUESTIONS AND ANSWERS RELATING TO THE RIGHTS OFFERING~~ABOUT THIS PROSPECTUS		1
PROSPECTUS SUMMARY		8	2
~~SUMMARY CONSOLIDATED FINANCIAL DATA~~THE OFFERING		13	9
~~SUMMARY OF THE RIGHTS OFFERING~~RISK FACTORS		15	11
~~RISK FACTORS~~		19
CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS		25	14
USE OF PROCEEDS		26	16
CAPITALIZATION		27	17
DILUTION		28	19
~~MARKET PRICE OF OUR COMMON STOCK AND RELATED STOCKHOLDER MATTERS~~EXECUTIVE COMPENSATION		29	21
~~DIVIDEND POLICY~~DESCRIPTION OF CAPITAL STOCK		30	26
~~THE RIGHTS OFFERING~~		30
~~MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES~~		39
DESCRIPTION OF SECURITIES WE ARE OFFERING		47	31
~~PLAN OF DISTRIBUTION~~PRINCIPAL STOCKHOLDERS		51	38
~~EXPERTS~~UNDERWRITING		52	42
~~LEGAL MATTERS~~EXPERTS		52	45
LEGAL MATTERS	45
WHERE YOU CAN FIND MORE INFORMATION		52	45
INCORPORATION BY REFERENCE		53	45

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ABOUT THIS PROSPECTUS

Unless the context otherwise requires, references in this prospectus to ~~"Onconova," "Onconova~~“Onconova,” “Onconova Therapeutics,~~" "Company," "we," "us"~~” “Company,” “we,” “us” and ~~"our"~~“our” refer to Onconova Therapeutics, Inc. and its consolidated subsidiaries. This prospectus is part of a registration statement that we have filed with the Securities and Exchange Commission, which we refer to as the SEC or the Commission, utilizing a registration process. It is important for you to read and consider all of the information contained in this prospectus and any applicable prospectus before making a decision whether to invest in ~~the common stock.~~our securities. You should also read and consider the information contained in the exhibits filed with our registration statement, of which this prospectus is a part, as described in ~~"Where~~“Where You Can Find More ~~Information"~~Information” in this prospectus.

You should rely only on the information contained in this prospectus and any applicable prospectus supplement, including the information incorporated by reference. WeNeither we nor the underwriter have ~~not~~ authorized anyone to provide you with different information. We are not offering to sell or soliciting offers to buy, and will not sell, any securities in any jurisdiction where it is unlawful. You should assume that the information contained in this prospectus or any prospectus supplement, as well as information contained in a document that we have previously filed or in the future will file with the SEC is accurate only as of the date of this prospectus, the applicable prospectus supplement or the document containing that information, as the case may be.
i
PROSPECTUS SUMMARY
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~~QUESTIONS AND ANSWERS RELATING TO THE RIGHTS OFFERING~~

The following ~~are examples of what we anticipate will be common questions about the Rights Offering. The answers are based on selected~~summary highlights certain information ~~included~~contained elsewhere in this prospectus. The following questions and answers do not contain all of the information that may be important to you and may not address all of the questions that you may have about the Rights Offering. This prospectus and the documents incorporated by reference ~~into this prospectus~~herein. This summary does not contain ~~more detailed descriptions of~~all the ~~terms and conditions of the Rights Offering and provide additional~~ information ~~about us and our business, including potential risks related to the Rights Offering, the Units offered hereby, and our business. We urge~~ you towill need in making your investment decision. You should carefully read this entire prospectus and the documents incorporated by reference ~~into this prospectus.~~
~~Why are we conducting the Rights Offering?~~
        ~~We are conducting the Rights Offering to raise additional capital:~~
•
to continue funding INSPIRE, our ongoing Phase 3 clinical trial of rigosertib IV in a population of patients with higher-risk myelodysplastic syndromes, or MDS, after failure of hypomethylating agent, or HMA, therapy, and

•
~~for general corporate purposes, including working capital, research and development, business development and operational purposes.~~
~~What is the Rights Offering?~~
        ~~We are distributing, at no charge, to record holders of our common stock and to holders of certain of our outstanding warrants who are entitled to participate in this Rights Offering pursuant~~herein. You should pay special attention to the terms of such warrants, non-transferable Subscription Rights to purchase Units at a price of $            per whole Unit. The Subscription Rights will not be tradable. Each Unit consists of            share of common stock and            Warrant representing the right to purchase one share of common stock at an exercise price of $            per share. Upon the effectiveness of the exercise of the Subscription Rights, the common stock and Warrants will immediately separate and will be issued as separate securities. We intend to apply to list the Warrants on NASDAQ under the symbol "ONTXW." You will receive one Subscription Right for each share of common stock or each share of common stock underlying the participating warrants that you owned as of 5:00 PM Eastern Time, on the Record Date. Each Subscription Right entitles the record holder or holder of a participating warrant to a Basic Subscription Right and an Over-Subscription Privilege.
~~What are the Basic Subscription Rights?~~
        For each whole share you owned or whole share underlying the participating warrants you owned as of the Record Date, you will receive one Basic Subscription Right, which gives you the opportunity to purchase            share of our common stock and to receive            Warrant to purchase one additional share of our common stock for a price of $            per Unit. For example, if you owned 100 shares of common stock as of the Record Date, you will receive 100 Subscription Rights and will have the right to purchase            shares of our common stock and            Warrants to purchase one additional share of our common stock for $            per whole Unit (or a total payment of $            ). You may exercise all or a portion of your Basic Subscription Rights or you may choose not to exercise any Basic Subscription Rights at all.
        If you are a record holder or a holder of participating warrants, the number of shares you may purchase pursuant to your Basic Subscription Rights is indicated on the enclosed Subscription Rights Statement. If you hold your shares in the name of a broker, dealer, bank, or other nominee who uses the services of the Depository Trust Company, or DTC, you will not receive a Subscription Rights Statement. Instead, DTC will issue one Subscription Right to your nominee record holder for each share of our common stock that you own as of the Record Date. If you are not contacted by your nominee, you should contact your nominee as soon as possible.

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~~What is the Over-Subscription Privilege?~~
        If you exercise your Basic Subscription Rights in full, you may also choose to exercise your Over-Subscription Privilege to purchase a portion of any Units that the other record holders and participating warrant holders do not purchase through the exercise of their Basic Subscription Rights. You should indicate on your Subscription Rights Statement, or the form provided by your nominee if your shares are held in the name of a nominee, how many additional Units you would like to purchase pursuant to your Over-Subscription Privilege.
        If sufficient Units are available, we will seek to honor your Over-Subscription request in full. If Over-Subscription requests exceed the number of Units available, however, we will allocate the available Units pro-rata among the record holders and participating warrant holders exercising the Over-Subscription Privilege in proportion to the number of shares of our common stock each of those record holders owned and the number of shares of our common stock underlying the participating warrants held by each of those warrant holders on the Record Date, relative to the number of shares owned and the number of shares underlying participating warrants held on the Record Date by all record holders and warrant holders exercising the Over-Subscription Privilege. If this pro-rata allocation results in any record holders or warrant holders receiving a greater number of Units than the record holder or warrant holder subscribed for pursuant to the exercise of the Over-Subscription Privilege, then such record holder or warrant holder will be allocated only that number of Units for which the record holder or warrant holder oversubscribed, and the remaining Units will be allocated among all other record holders and warrant holders exercising the Over-Subscription Privilege on the same pro rata basis described above. The proration process will be repeated until all Units have been allocated. See "The Rights Offering—Limitation on the Purchase of Units" for a description of certain limitations on purchase.
        To properly exercise your Over-Subscription Privilege, you must deliver to the Subscription Agent the subscription payment related to your Over-Subscription Privilege before the Rights Offering expires. See "The Rights Offering—The Subscription Rights—Over-Subscription Privilege." To the extent you properly exercise your Over-Subscription Privilege for an amount of Units that exceeds the number of unsubscribed Units available to you, any excess subscription payments will be returned to you as soon as practicable after the expiration of the Rights Offering, without interest or penalty.
        ~~Wells Fargo Bank, N.A., our Subscription Agent for the Rights Offering, will determine the Over-Subscription allocation based on the formula described above.~~
~~What are the terms of the Warrants?~~
        Each Warrant entitles the holder to purchase one share of common stock at an exercise price of $            per share from the date of issuance through its expiration on                    . The Warrants will be exercisable by paying the exercise price in cash, or, solely during any period when a registration statement for the exercise of the Warrants is not in effect, exercisable on a cashless basis. We may redeem the Warrants for $            per Warrant if our common stock closes above $            per share for 10 consecutive trading days.
~~Will fractional shares be issued upon exercise of Subscription Rights or upon the exercise of Warrants?~~
        No. We will not issue fractional shares of common stock in the Rights Offering. Rights holders will only be entitled to purchase a number of Units representing a whole number of shares of common stock, rounded down to the nearest whole number of Units a holder would otherwise be entitled to purchase. Any excess subscription payments received by the Subscription Agent will be returned as soon as practicable after expiration of the Rights Offering, without interest or penalty. Similarly, no fractional shares of common stock will be issued in connection with the exercise of a Warrant. If, upon

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exercise of a Warrant, the holder thereof would be entitled to receive a fractional share of common stock, upon exercise, the holder will only be entitled to receive a whole number of shares of common stock, rounded down to the nearest whole number.
~~What effect will the Rights Offering have on our outstanding common stock?~~
        After our one-for-ten reverse stock split on May 31, 2016, approximately 2,740,104 shares of our common stock were outstanding, along with participating warrants to purchase approximately 96,842 shares of common stock, in each case subject to final adjustments for treatment of fractional share interests. Based on the foregoing, and assuming no other transactions by us involving our common stock prior to the expiration of the Rights Offering, if the Rights Offering is fully subscribed, approximately             shares of our common stock will be issued and outstanding and Warrants to purchase approximately            additional shares of our common stock will be outstanding (excluding the currently outstanding warrants). The exact number of shares and Warrants that we will issue in this Rights Offering will depend on the number of Units that are subscribed for in the Rights Offering.
~~How was the Subscription Price determined?~~
        ~~In determining the Subscription Price, the pricing committee of our board of directors is expected to consider, among other things, the following factors:~~
•
~~the current and historical trading prices of our common stock;~~

•
~~the price at which shareholders might be willing to participate in the Rights Offering;~~

•
~~the value of the Warrant being issued as a component of the Unit;~~

•
~~our need for additional capital and liquidity;~~

•
~~the cost of capital from other sources; and~~

•
comparable precedent transactions, including the percentage of shares offered, the terms of the subscription rights being offered, the subscription price and the discount that the subscription price represented to the immediately prevailing closing prices for those offerings.
        In conjunction with the review of these factors, our pricing committee will also review our history and prospects, including our past and present earnings and cash requirements, our prospects for the future, the outlook for our industry and our current financial condition. Our pricing committee believes that the Subscription Price should be designed to provide an incentive to our current shareholders and holders of the participating warrants to participate in the Rights Offering and exercise their Basic Subscription Right and their Over-Subscription Privilege.
        The Subscription Price does not necessarily bear any relationship to any established criteria for value. You should not consider the Subscription Price as an indication of actual value of our company or our common stock. We cannot assure you that the market price of our common stock will not decline during or after the Rights Offering. You should obtain a current price quote for our common stock before exercising your Subscription Rights and make your own assessment of our business and financial condition, our prospects for the future, and the terms“Risk Factors” section of this ~~Rights Offering. Once made, all exercises of Subscription Rights are irrevocable.~~
~~Am I required to exercise all of the Basic Subscription Rights I receive in the Rights Offering?~~
        No. You may exercise any number of your Basic Subscription Rights, or you may choose not to exercise any Basic Subscription Rights. If you do not exercise any Basic Subscription Rights, the number of shares of our common stock or number of shares underlying our warrants you own will not change. However, if you choose to not exercise your Basic Subscription Rights in full, your

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~~proportionate ownership interest in our company will decrease. If you do not exercise your Basic Subscription Rights in full, you will not be entitled to exercise your Over-Subscription Privilege.~~
~~How soon must I act to exercise my Subscription Rights?~~
        If you received a Subscription Rights Statement and elect to exercise any or all of your Subscription Rights, the Subscription Agent must receive your completed and signed Subscription Rights Statement and payment for both your Basic Subscription Rights and any Over-Subscription Privilege you elect to exercise, including final clearance of any uncertified check, before the Rights Offering expires on                    , 2016, at 5:00 PM Eastern Time. If you hold your shares in the name of a broker, dealer, custodian bank, or other nominee, your nominee may establish a deadline before the expiration of the Rights Offering by which you must provide it with your instructions to exercise your Subscription Rights, along with the required subscription payment.
~~May I transfer my Subscription Rights?~~
        No. The Subscription Rights may be exercised only by the shareholders or participating warrant holders to whom they are distributed, and they may not be sold, transferred, assigned or given away to anyone else, other than by operation of law. As a result, a Subscription Rights Statement may be completed only by the shareholder or participating warrant holder who receives the statement. The Subscription Rights will not be listed for trading on any stock exchange or market.
~~Will our directors and executive officers participate in the Rights Offering?~~
        To the extent they hold common stock as of the Record Date, our directors and executive officers will be entitled to participate in the Rights Offering on the same terms and conditions applicable to other Rights holders.
~~Has the board of directors made a recommendation to shareholders and warrant holders regarding the Rights Offering?~~
        No. Our board of directors is not making a recommendation regarding your exercise of the Subscription Rights. Stockholders and holders of the participating warrants who exercise Subscription Rights will incur investment risk on new money invested. We cannot predict the price at which our shares of common stock will trade after the Rights Offering. On June 1, 2016, the closing price of our common stock was $            per share. The market price for our common stock may be above the Subscription Price or may be below the Subscription Price. If you exercise your Subscription Rights, you may not be able to sell the underlying shares of our common stock or Warrants in the future at the same price or a higher price. You should make your decision based on your assessment of our business and financial condition, our prospects for the future, the terms of the Rights Offeringprospectus and the ~~information contained in this prospectus. See "Risk Factors" for discussion of some of the risks involved in investing in our securities.~~
~~How do I exercise my Subscription Rights?~~
        If you are a shareholder of record (meaning you hold your shares of our common stock in your name and not through a broker, dealer, bank, or other nominee) or a holder of the participating warrants and you wish to participate in the Rights Offering, you must deliver a properly completed and signed Subscription Rights Statement, together with payment of the Subscription Price for both your Basic Subscription Rights and any Over-Subscription Privilege you elect to exercise, to the Subscription Agent before 5:00 PM Eastern Time, on                    , 2016. If you are exercising your Subscription Rights through your broker, dealer, bank, or other nominee, you should promptly contact your broker, dealer, bank, or other nominee and submit your subscription documents and payment for the Units

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~~subscribed for in accordance with the instructions and within the time period provided by your broker, dealer, bank or other nominee.~~
~~What if my shares are held in "street name"?~~
        If you hold your shares of our common stock in the name of a broker, dealer, bank, or other nominee, then your broker, dealer, bank, or other nominee is the record holder of the shares you own. The record holder must exercise the Subscription Rights on your behalf. Therefore, you will need to have your record holder act for you.
        If you wish to participate in this Rights Offering and purchase Units, please promptly contact the record holder of your shares. We will ask the record holder of your shares, who may be your broker, dealer, bank, or other nominee, to notify you of this Rights Offering.
~~What form of payment is required?~~
        ~~You must timely pay the full Subscription Price for the full number of Units you wish to acquire pursuant to the exercise of Subscription Rights by delivering to the Subscription Agent a:~~
•
~~cashier's check drawn on a U.S. bank; or~~

•
~~wire transfer.~~
        If you send a payment that is insufficient to purchase the number of Units you requested, or if the number of Units you requested is not specified in the forms, the payment received will be applied to exercise your Subscription Rights to the fullest extent possible based on the amount of the payment received.
~~When will I receive my new shares of common stock and Warrants?~~
        The Subscription Agent will arrange for the issuance of the common stock and Warrants as soon as practicable after the expiration of the Rights Offering, payment for the Units subscribed for has cleared, and all prorating calculations and reductions contemplated by the terms of the Rights Offering have been effected. All shares and Warrants that you purchase in the Rights Offering will be issued in book-entry, or uncertificated, form meaning that you will receive a direct registration (DRS) account statement from our transfer agent reflecting ownership of these securities if you are a holder of record of shares or warrants. If you hold your shares in the name of a broker, dealer, bank, or other nominee, DTC will credit your account with your nominee with the securities you purchase in the Rights Offering.
~~After I send in my payment and Subscription Rights Statement to the Subscription Agent, may I cancel my exercise of Subscription Rights?~~
        No. Exercises of Subscription Rights are irrevocable unless the Rights Offering is terminated, even if you later learn information that you consider to be unfavorable to the exercise of your Subscription Rights. You should not exercise your Subscription Rights unless you are certain that you wish to purchase Units at the Subscription Price.
~~How much will our company receive from the Rights Offering?~~
        Assuming that all            Units are sold in the Rights Offering, we estimate that the net proceeds from the Rights Offering will be approximately $             million, based on the Subscription Price of $            per Unit, after deducting fees and expenses payable to the dealer-manager, and after deducting other expenses payable by us and excluding any proceeds received upon exercise of any Warrants issued in the Rights Offering.

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~~Are there risks in exercising my Subscription Rights?~~
        Yes. The exercise of your Subscription Rights involves risks. Exercising your Subscription Rights involves the purchase of additional shares of our common stock and Warrants to purchase common stock and you should consider this investment as carefully as you would consider any other investment. We cannot assure you that the market price of our common stock will exceed the Subscription Price, nor can we assure you that the market price of our common stock will not further decline during or after the Rights Offering. We also cannot assure you that you will be able to sell shares of our common stock or Warrants purchased in the Rights Offering at a price equal to or greater than the Subscription Price. In addition, you should carefully consider the risks described under the heading "Risk Factors" for discussion of some of the risks involved in investing in our securities.
~~Can the board of directors terminate or extend the Rights Offering?~~
        Yes. Our board of directors may decide to terminate the Rights Offering at any time and for any reason before the expiration of the Rights Offering. We also have the right to extend the Rights Offering for additional periods in our sole discretion. We do not presently intend to extend the Rights Offering. We will notify shareholders and warrant holders if the Rights Offering is terminated or extended by issuing a press release.
~~If the Rights Offering is not completed or is terminated, will my subscription payment be refunded to me?~~
        Yes. The Subscription Agent will hold all funds it receives in a segregated bank account until completion of the Rights Offering. If we do not complete the Rights Offering, all subscription payments received by the Subscription Agent will be returned as soon as practicable after the termination or expiration of the Rights Offering, without interest or penalty. If you own shares in "street name," it may take longer for you to receive your subscription payment because the Subscription Agent will return payments through the record holder of your shares.
~~How do I exercise my Rights if I live outside the United States?~~
        The Subscription Agent will hold Subscription Rights Statements for shareholders having addresses outside the United States. To exercise Subscription Rights, foreign shareholders must notify the Subscription Agent and timely follow other procedures described in the section entitled "The Rights Offering—Foreign Shareholders."
~~What fees or charges apply if I purchase shares in the Rights Offering?~~
        We are not charging any fee or sales commission to issue Subscription Rights to you or to issue shares or Warrants to you if you exercise your Subscription Rights. If you exercise your Subscription Rights through a broker, dealer, custodian bank, or other nominee, you are responsible for paying any fees your broker, dealer, bank, or other nominee may charge you.
~~What are the U.S. federal income tax consequences of exercising my Subscription Rights?~~
        For U.S. federal income tax purposes, we do not believe you should recognize income or loss in connection with the receipt or exercise of Subscription Rights in the Rights Offering. You should consult your tax advisor as to the tax consequences of the Rights Offering in light of your particular circumstances. For a more detailed discussion, see "Material U.S. Federal Income Tax Consequences."

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~~To whom should I send my forms and payment?~~
        If your shares are held in the name of a broker, dealer, bank, or other nominee, then you should send your subscription documents and subscription payment to that broker, dealer, bank, or other nominee. If you are the record holder or a holder of participating warrants, then you should send your Subscription Rights Statement and payment of your subscription price to the Subscription Agent hand delivery, first class mail or courier service to:
~~By Mail~~:
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~~Whom should I contact if I have other questions?~~
        ~~If you have other questions or need assistance, please contact the dealer-manager for the Rights Offering:~~
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~~Attention Syndicate Department~~
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~~Who is the dealer-manager?~~
        Maxim Group LLC will act as dealer-manager for the Rights Offering. Under the terms and subject to the conditions contained in the dealer-manager agreement, the dealer-manager will use its best efforts to solicit the exercise of Subscription Rights. We have agreed to pay the dealer-manager certain fees for acting as dealer-manager and to reimburse the dealer-manager for certain out-of-pocket expenses incurred in connection with this offering. The dealer-manager is not underwriting or placing any of the Subscription Rights or the Units, shares of common stock or Warrants being issued in this offering, and do not make any recommendation with respect to such Subscription Rights (including with respect to the exercise or expiration of such Subscription Rights), Units, shares of common stock or Warrants. See "Plan of Distribution" for a discussion of the fees and expenses to be paid to the dealer-manager in connection with this Rights Offering.

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~~PROSPECTUS SUMMARY~~
The items in the following summary are described in more detail later in this prospectus. This summary provides an overview of selected information and does not contain all of the information you should consider. Before investing in our common stock, you should read the entire prospectus carefully, including the information set forth under the headings "Risk Factors "and the consolidated financial statements and ~~related notes included or~~other information incorporated by reference in this prospectus.

~~OUR COMPANY~~
Our Business
~~Onconova Therapeutics, Inc., sometimes referred to as "we" or the "Company," is~~
Overview
We are a clinical-stage biopharmaceutical company focused on discovering and developing novel small molecule ~~drug~~product candidates primarily to treat cancer. Using our proprietary chemistry platform, we have created ~~an extensive~~a library of targeted ~~anti-cancer~~ agents designed to work against cellular pathways important to cancer cells. We believe that the ~~drug~~product candidates in our pipeline have the potential to be efficacious in a variety of cancers. We have one Phase 3 clinical-stage product candidate and two other clinical-stage product candidates (one of which is being developed for treatment of acute radiation syndromes) and several preclinical programs. Substantially all of our current effort is focused on our lead product candidate, rigosertib. Rigosertib is being tested in ~~both~~an intravenous ~~and oral formulations~~formulation as a single agent, and ~~the~~an oral formulation ~~is also being tested~~ in combination with azacitidine, in clinical trials for patients with higher-risk myelodysplastic syndromes ~~or MDS, and related cancers.~~(“MDS”).

In December 2015, we enrolled the first patient ininto our INSPIRE trial, a randomized controlled Phase 3 clinical trial of intravenous rigosertib IV(“rigosertib IV”) in a population of patients with higher-risk MDS after failure of hypomethylating agent ~~or HMA,~~(“HMA”) therapy. ~~The trial, which we refer to as INSPIRE, is expected to enroll approximately 225 patients at more than 100 sites globally.~~ The primary endpoint of INSPIRE is overall ~~survival, and an~~survival. An interim analysis ~~is anticipated. We~~of the trial was performed in January 2018 and we anticipate reporting topline data from the INSPIRE trial in ~~2018.~~
        In the first ~~quarter~~half of ~~2016, we took significant actions to conserve cash, including reduction in personnel~~2019.
Our net losses were $24.1 million and ~~expenditures. While we will continue to take cash conservation actions where appropriate, our costs will increase in subsequent quarters as more INSPIRE sites open and more patients enroll in~~$19.7 million for the INSPIRE trial. We believe that our cash and cash equivalents, prior to our receipt of any proceeds from this Rights Offering, together with anticipated contractual cost-sharing payments from Baxalta GmbH, or Baxalta, for a portion of the INSPIRE trial costs, will be sufficient to fund our ongoing trials and operations into the first quarter of 2017.
        We are exploring various sources of funding for continued development of rigosertib in MDS and acute myelogenous leukemia, or AML, as well as our ongoing operations. Completion of the Rights Offering is not subject to us raising a minimum offering amount and therefore the net proceeds from the Rights Offering may be insufficient to meet our objectives. Even if we sell all of the Units subject to the Rights Offering, we will need to obtain additional financing in the future in order to fully fund rigosertib or any other product candidates through the regulatory approval process.
        Due to our ongoing losses and our accumulated deficit in combination with other factors, the opinion of our independent registered public accounting firm on our audited consolidated financial statements for our fiscal yearyears ended December 31, ~~2015 contains~~2017 and 2016, respectively. As of December 31, 2017, we had an ~~explanatory paragraph regarding substantial doubt about our ability to continue as a going concern. Even if we sell all~~accumulated deficit of the Units subject to the Rights Offering, if we are unable to obtain sufficient additional funding, through future financings or through strategic and collaborative transactions and arrangements, we may not have sufficient resources to complete our INSPIRE trial and we may continue to delay, scale-back or eliminate certain of our planned research, drug discovery and development activities and certain other aspects of our operations, or we may not be able to continue as a going concern.

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~~Rigosertib~~$362.3 million.

Rigosertib
Rigosertib is a small molecule that ~~inhibits~~we believe blocks cellular signaling by ~~acting as a Ras mimetic.~~targeting RAS effector pathways. This is believed to be mediated by the ~~binding~~interaction of rigosertib to the ~~Ras-binding~~RAS-binding domain ~~or RBD,~~(“RBD”), found in many ~~Ras~~RAS effector proteins, including the Raf and PI3K kinases. ~~This~~We believe this mechanism of action provides a new approach to block the interactions between ~~Ras~~RAS and its targets containing RBD sites. Rigosertib is currently being tested in clinical trials as a single agent, and in combination with azacitidine, in ~~clinical trials of~~ patients with ~~MDS and related cancers.~~MDS. We have enrolled more than ~~1,200~~1,300 patients in rigosertib clinical ~~trials.~~trials for MDS and other conditions. We ~~are~~were a party to a license and development agreement with Baxalta (as defined below), which granted Baxalta certain rights to commercialize rigosertib in Europe. The Baxalta agreement ~~is scheduled to terminate~~was terminated on August 30, 2016, at which time the European rights ~~will revert~~reverted to us at no cost. We are ~~also~~ party to a collaboration agreement with ~~SymBio Pharmaceuticals Limited, or~~ SymBio, which grants SymBio certain rights to commercialize rigosertib in Japan and Korea. We are party to a license agreement with Pint Pharma International SA (“Pint”), which grants Pint certain rights to commercialize rigosertib in Latin America. We have retained development and commercialization rights to rigosertib in the rest of the world, including in the United States and Europe, although we ~~will~~could consider licensing commercialization rights to other territories as ~~a source of~~we continue to seek additional funding. Previously we were a party to a license and development agreement with Baxalta (as defined below), which granted Baxalta certain rights to commercialize rigosertib in Europe. The Baxalta agreement was terminated on August 30, 2016, at which time the European rights reverted to us at no cost.
The table below summarizes our rigosertib clinical stage programs.
Rigosertib IV for higher-risk MDS
We are developing an IV version of rigosertib for the treatment of higher-risk MDS following the failure of HMA therapy. In early 2014, we announced topline survival results from our ~~"ONTIME"~~“ONTIME” trial, a multi-center Phase 3 clinical trial of rigosertib IV as a single ~~agent.~~agent versus best supportive care including low dose Ara-C. The ONTIME trial did not meet its primary endpoint of an improvement in overall survival in the intent-to-treat population, although improvements in median overall survival were observed in various pre-specified and exploratory subgroups of higher-risk MDS patients. As a result, additional clinical work is on-going.

During 2014 and 2015, we held meetings with the U.S. Food and Drug Administration ~~or FDA,~~(“FDA”), European Medicines Agency ~~or EMA,~~(“EMA”), and several European national regulatory authorities to discuss and seek guidance on a path for approval of rigosertib IV in higher-risk MDS patients whose disease had failed HMA therapy. After discussions with the FDA and EMA, we refined our patient eligibility criteria by defining what we believe to be a more homogenous patient population. After regulatory feedback, input from key opinion leaders in the U.S. and Europe and based on learnings from the ONTIME study, we designed a new randomized controlled Phase 3 trial, referred to as ~~INSPIRE, with overall survival as a primary endpoint.~~INSPIRE. The INSPIRE trial is enrolling higher-risk MDS patients under 8082 years of age who have progressed on, relapsed, or failed to respond to, previous treatment with HMAs within ~~the first~~ nine months or nine cycles over the course of one year after initiation of HMA therapy, and had their last dose of HMA within six months prior to enrollment in the trial. Patients are randomized to either rigosertib with best supportive care, or the physician’s choice of therapy with best supportive care. The primary endpoint of this study is the sequential analysis of overall survival of all randomized patients in the intent-to-treat (“ITT”) population and an interim analysis is anticipated. This randomized trial of approximately 225 patients is expected to be conducted at more than 100 sites globally. In August 2015, we submitted an updated investigational new drug application, or IND, to the FDA, and in August 2015 we submitted Clinical Trial Applications, or CTAs, with the United Kingdom, German and Austrian regulatory authorities for IV rigosertib as a treatment for higher-risk MDS after failure of HMA therapy. The first CTA has been cleared by the Medicines and Healthcare products Regulatory Agency.International Prognostic Scoring System- Revised (IPSS-R) Very High Risk subgroup. The first patient in the INSPIRE trial was enrolled at the MD Anderson Cancer Center in December 2015, ~~and, as of May 18, 2016, 59 clinical sites are open (43 in~~ the ~~U.S. and Europe and 16 in Japan) and can recruit patients. The~~ first patient in Europe was enrolled onin March, ~~18,~~2016, and the first patient in Japan was enrolled in July, 2016.
Enrollment for the INSPIRE Phase 3 trial for second-line higher-risk MDS patients is highly selective with stringent entry criteria as outlined above. Currently, the INSPIRE study has opened approximately 175 trial sites in 22 countries across four continents, and has enrolled more than 170 patients. Our partner, SymBio Pharmaceuticals, has opened more than 30 sites in Japan. The selection of countries and trial sites is carefully undertaken to ensure availability of appropriate patients meeting eligibility criteria. Since these criteria are purposely designed to be narrow and selective, extensive site screening and education is integral to our plan. At launch, the INSPIRE trial was expected to enroll 225 patients and the outcome is measured by overall survival.
The INSPIRE trial included a pre-planned interim analysis triggered by 88 events (deaths), which occurred in December 2017. The statistical analysis plan (“SAP”) for the INSPIRE trial featured an adaptive trial design,
permitting several options following the interim analysis, which included continuation of the trial as planned, discontinuation of the trial for futility or safety, trial expansion using pre-planned sample size re-estimation, and trial continuation for only the pre-defined treatment subgroup of patients classified as Very High Risk (“VHR”) based on the IPSS-R.
After review of the interim data, in January 2018 the Independent Data Monitoring Committee (“DMC”) recommended continuation of the trial with a one-time expansion in enrollment, using a pre-planned sample size re-estimation, consistent with the SAP. As recommended by the DMC, the expanded INSPIRE study will continue to enroll eligible patients based on the current trial criteria of the overall ITT population and will increase enrollment by adding 135 patients to the original target to reach a total enrollment of 360 patients, with the aim of increasing the power of the trial. Due to the adaptive trial design and the DMC’s assessment, the INSPIRE trial will continue to analyze both the ITT and the VHR population for the primary endpoint of overall survival. The design of the trial with the expanded study enrollment will be identical to the current study design and will include the sequential analysis of the overall survival endpoint in the ITT population and if required the pre-specified VHR subgroup. The Company remains blinded to the specific interim analysis results. We anticipate reporting topline data from the INSPIRE trial in the first half of 2019.
Safety and Tolerability of rigosertib in MDS and other hematologic malignancies
A comprehensive analysis of IV and rigosertib oral safety in patients with Myelodysplastic Syndromes (MDS) and Acute Myeloid Leukemia (AML) was presented in December 2016 at the American Society of Hematology (ASH) Annual Meeting. The most commonly reported treatment-emergent adverse events (TEAEs) in > 10% of patients with MDS/AML (n= 335) receiving rigosertib intravenous (IV) monotherapy were fatigue (33%), nausea (33%), diarrhea (27%), constipation (25%), anaemia (24%) and pyrexia (24%). The most common > Grade 3 AEs were anaemia (21%), febrile neutropenia (13%), pneumonia (12%) and thrombocytopenia (11%). The most common serious AEs were febrile neutropenia (10%), pneumonia (9%), and sepsis (7%). The most common AEs leading to discontinuation of IV rigosertib were sepsis and pneumonia (3% each).
Rigosertib oral in combination with azacitidine for higher-risk MDS ~~and AML~~

We ~~have completed enrollment~~are developing rigosertib oral for use in combination with azacitidine prior to treatment with HMA therapy for higher risk MDS. In December 2016, at the American Society of Hematology (ASH) Annual Meeting, we presented Phase 1/2 data from the initial portion of an ~~open label Phase 1/2 clinical~~ongoing rigosertib oral and azacitidine combination trial ~~testing~~in higher-risk MDS. 33 of 40 MDS patients enrolled were evaluable for response at the time of the analysis. The median age of patients was 66, with 73% being male. The IPSS-R distribution was: 7.5% Low, 12.5% Intermediate, 37.5% High, 32.5% Very High and 10% unknown. 76% of patients responded per 2006 International Working Group (IWG) criteria. Responses were as follows:
Response per IWG 2006

Overall
Evaluable
(N=33)

No prior
HMA
(N-20)

Prior
HMA
(N=13)

Complete remission (CR)

8(24
)%
7(35
)%
1(8
)%
Marrow CR + hematologic improvement

10(30
)%
6(30
)%
4(31
)%
Marrow CR alone

6(18
)%
3(15
)%
3(23
)%
Hematologic improvement alone

1(3
)%
1(5
)%
0

Stable disease

8(24
)%
3(15
)%
5(38
)%
Overall IWG response

25(76
)%
17(85
)%
8(62
)%
Clinical benefit response

19(58
)%
14(70
)%
5(38
)%
The median duration of response was 8 months for CR, 12.3 months for marrow CR.
Safety/Tolerability of the Combination:
Based upon a comprehensive analysis of patients receiving oral rigosertib in combination with azacitidine that was presented in 2016, the combination of rigosertib oral and azacitidine was well tolerated. The most common TEAEs in > 10% of patients with MDS/AML (n=54) receiving rigosertib oral and azacitidine were nausea (41%), fatigue (39%), diarrhea (37%), constipation (37%) and dysuria (28%). The most common serious AEs were pneumonia (11%) and febrile neutropenia (7%). The most common AEs leading to discontinuation were AML (4%) and pneumonia (4%).
Next steps for rigosertib oral in combination with azacitidine for higher-risk MDS
Following an end of Phase 2 meeting with the ~~approved~~Food and Drug Administration (FDA) in September 2016, we began development of a Phase 3 protocol. The Phase 3 trial will be designed as a global 1:1 randomized, placebo-controlled trial of rigosertib oral plus azacitidine compared to azacitidine plus oral placebo. Based on the results of the Phase 1/2 Study, full dose of ~~injectable~~ azacitidine for patients with higher-risk MDS and AML. This study is based on our published preclinical data demonstrating synergistic activity of this combination. The Phase 2 portion of the trial was designed to assess whether treatment with rigosertib,will be used in combination with rigosertib oral, as defined in the ~~approved~~product insert for azacitidine. The patient population studied in this trial will be first-line (HMA naïve) higher-risk MDS patients. The primary endpoint for assessment of efficacy will be the composite Response Rate of complete remission (CR) + partial remission (PR,) as per the IWG 2006 Response Criteria. The trial will be under the review of a DMC. Formal FDA review may be sought via the Special Protocol Assessment (SPA) mechanism. We will not commence the Phase 3 trial without additional financing.
While the Phase 3 trial is being designed, we have expanded the Phase 1/2 trial cohort by enrolling 45 additional patients. Under a protocol expansion, we are using the expanded cohorts to explore dose optimization by increasing the dose of ~~injectable~~rigosertib oral to a total of 1120 mg in combination with full dose azacitidine ~~reduces~~and varying the ~~number~~dose administration scheme of ~~bone marrow blasts, improves peripheral blood counts~~rigosertib oral (560 mg 9 AM/560 mg 9 PM and ~~can resensitize~~840 mg 9 AM/280 mg 9 PM) to identify an optimal dose and schedule. During this expansion, we also instituted risk-mitigation strategies, as further described below, in order to address a prior urinary adverse event of interest, hematuria. After amendments were filed with the ~~marrow blast cells to azacitidine for patients who were previously exposed to azacitidine. Patient enrollment in~~regulatory agencies, we started the

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~~Phase 2 portion~~ expansion phase of this trial ~~was completed~~ in the ~~fourth quarter~~U.S. sites that participated in the initial trial. Since the trial initiation, we have added additional US sites to complete enrollment of ~~2015~~the expanded trial. The first patient was enrolled in April 2017 and ~~interim data were summarized by way~~as of ~~an oral presentation~~April 2017 half of all of the planned patients have been enrolled in the expansion trial; and the trial is ongoing.
In March 2018, at the ~~ASH Annual Meeting~~6th International Bone Marrow Failure Disease Symposium, we presented data on the incidence of hematuria in ~~December 2015.~~
        ~~The Phase 2 trial included both front-line~~37 higher-risk MDS patients ~~(that is, patients not previously treated~~receiving rigosertib oral in combination with HMAs) and MDS patients whose disease had failed prior HMA therapy (second-line patients). The presentation at ASH included results from a total of 37 MDS patients treated with the recommended Phase 2 dose of oral rigosertib (560 mg AM/280 mg PM) plus the full standard dose of injectable azacitidine. The combination of oral rigosertib and azacitidine ~~was generally well tolerated, with a median duration of treatment of four months (range 1 to 27 months).~~
        At the time of the presentation, 30 MDS patients were evaluable for efficacy assessment per 2006 IWG, criteria. Twenty-three of 30 patients (77%) responded to the combination therapy, including six patients who had complete remissions. Hematologic improvement was observed in 13 of 26 patients that were evaluable for thisas part of the ~~analysis. Notably, 16~~Phase 1/2 expanded cohort . In the first part of ~~19 (84%) HMA-naïve patients had a response~~the Phase 1/2 study, prior to the ~~combination therapy~~study expansion, of 42 patients studied with oral rigosertib 840 mg total and 7azacitidine, the incidence of ~~11 (64%)~~hematuria was 48%. In 37 patients ~~whose disease had previously failed HMAs responded. As~~studied with oral rigosertib 1120 mg total and azacitidine in the Phase 1/2 expanded cohort, with the use of ~~December 2015,~~risk-mitigating strategies to minimize hematuria, the ~~median duration~~incidence of ~~these responses had not yet been reached. Additional data collection for efficacy and safety continues for the patients remaining on~~hematuria was 11%. The study ~~and may impact the final results~~is ongoing.    The risk-mitigating strategies employed are as follows:
Risk-Mitigation Strategies to Minimize Hematuria:
2nd RIGO dose must be administered at 3 PM (±1 hour) at least 2 hours after lunch to avoid a nocturnal bladder dwell time
Oral hydration of at least two liters of fluid per day is encouraged
Mandatory bladder emptying prior to bedtime
Urine pH approximately 2 hrs after AM dose. Sodium bicarbonate suggested administration of 650 TID if pH tests < 7.5
The comparison of the ~~trial. Updated data~~hematuria results from the ~~Phase 2 Trial is expected to be~~two parts of this study are presented below:
Hematuria Comparison Between Rigosertib Combination Therapy Parts 1 and 2:
All Patients on Combination Part 1 (Rigosertib 840 mg total & Azacitidine)
42
Patients with hematuria
20 (48%)
Patients with grade 1 or 2 hematuria
17 (40%)
Patients with grade >3 hematuria
5 (12%)
All Patients on Combination Part 2 (Rigosertib 1120 mg total) & Azacitidine) with risk-mitigation strategies
37
Patients with hematuria
4 (11%)
Patients with grade 1 or 2 hematuria
4 (11%)
Patients with grade >3 hematuria
0 (0%)
In June 2017, at the Congress of the European Hematology Association Meeting, we updated the data from the Phase 1/2 trial and highlighted results in ~~June 2016.~~AML patients included in this study. Response data was presented on eight evaluable patients with AML who were tested with the rigosertib and azacitidine combination. For the eight evaluable patients with AML, the combination was well tolerated and the safety profile was similar to single-agent azacitidine, based on safety information in the azacitidine FDA approved label. Based on the presented results of the combination studies, the authors concluded that continued study in AML was warranted. We will not commence further development of rigosertib oral in combination with azacitidine for AML without additional financing.
Upon completion of our Phase 1/2 study, we will submit the study results to the applicable regulatory authorities. The final results of this study may differ from the results presented above and the applicable regulatory authorities may not agree with our analyses. We will not commence the Phase 3 trial of oral rigosertib in combination with azacitidine for higher-risk MDS or AML without additional financing.
Rigosertib oral for lower-risk MDS
We are also developing rigosertib oral as a single agent treatment for lower risk MDS. Higher-risk MDS patients suffer from a shortfall in normal circulating blood cells, or cytopenias, as well as elevated levels of cancer cells, or blasts in their bone marrow and sometimes in their peripheral blood ~~whereas lower-risk~~with a significant rate of transformation to acute leukemia. Lower-risk MDS patients suffer mainly from cytopenias, that is low levels of red blood cells, white blood cells ~~and/~~or platelets. Thus, lower-risk MDS patients depend on transfusions and growth factors or other therapies to improve their low blood ~~counts.~~counts; but have a lower rate of acute leukemic transformation.

We have explored single agent rigosertib oral as a treatment for lower-risk MDS in two Phase 2 clinical trials, 09-05 and 09-07. In December ~~2013,~~2017, we presented data at the Annual ASH Meeting from the 09-05 Phase 2 trial. This data demonstrated a 44% rate of achieving transfusion independence in the cohort of Lower -risk MDS patients treated with rigosertib oral at a dose of 560 mg BID (1120 mg over 24 hrs). To date, Phase 2 clinical data ~~have shown encouraging signs of efficacy~~has indicated that further study of single agent rigosertib oral ~~rigosertib~~ in transfusion-dependent, lower-risk MDS ~~patients.~~patients is warranted. Rigosertib has been generally well tolerated, except for urinary side effects at higher dose levels. ~~Additional~~Future clinical ~~testing~~ trials
will be needed to evaluate dosing and schedule modifications and their impact on efficacy and ~~toxicity~~safety results of rigosertib oral ~~rigosertib~~ in lower-risk MDS patients.

Data presented from the 09-05 trial also suggested the potential of a genomic methylation assessment of bone marrow cells to prospectively identify lower-risk MDS patients likely to respond to ~~oral rigosertib.~~rigosertib oral. We therefore expanded the 09-05 trial by adding an additional cohort of 20 patients to advance the development of this genomic methylation test. ~~Enrollment~~To date, a biomarker which would predict response has not been identified. Further testing and development of rigosertib oral for lower-risk MDS will be required. We will not commence further development of rigosertib oral for lower-risk MDS without additional financing.
Safety and Tolerability of rigosertib oral in MDS and other hematologic malignancies
As presented at the December 2016 ASH Annual Meeting, rigosertib oral as a monotherapy was evaluated in four Phase 1 and 2 studies in MDS and other hematologic malignancies. One study is completed and a clinical study report is available. The most common TEAEs in > 10% of patients with MDS/AML (n=168) were pollakiuria (increased urinary frequency) (35%), fatigue (32%), diarrhea (26%), dysuria (29%) and haematuria (24%). The most common > Grade 3 AEs were anaemia (17%), thrombocytopenia (5%), haematuria (4%) and urinary tract infection (4%). The most common serious AE was pneumonia (6%). The most common AEs leading to discontinuation of patients receiving rigosertib oral as monotherapy were dysuria (8%), urinary tract pain (7%), haematuria (5%) and urinary frequency (5%).
In addition to the above described clinical trials, we are continuing the preclinical and chemistry, manufacturing, and control work for IV and rigosertib oral.
Rare Disease Program in “RASopathies”
Based on new mechanism of action data published last year, we are initiating a collaborative development program focusing on a group of rare diseases with a well-defined molecular basis in expression or defects involving the Ras Effector Pathways. Since “RASopathies” are rare diseases affecting young children, we are embarking on a multifaceted collaborative program involving patient advocacy, government and academic organizations. The RASopathies are a group of rare diseases which share a well-defined molecular basis in expression or defects involving Ras Effector Pathways. They are usually caused by germline mutations in genes that alter the RAS subfamily and mitogen-activated protein kinases that control signal transduction, and are among the most common genetic syndromes. Together, this ~~expansion cohort~~group of diseases can impact more than 1 in 1000 individuals, according to RASopathiesNet.
In January 2018, we entered into a Cooperative Research and Development Agreement (CRADA) with the National Cancer Institute (NCI), part of the National Institutes of Health (NIH). Under the terms of the CRADA, the NCI will conduct research, including preclinical laboratory studies and a clinical trial, on rigosertib in pediatric cancer associated RASopathies.
As part of the CRADA, we will provide rigosertib supplies and initial funding towards non-clinical studies. The NCI will fund the majority of the research, including the cost of the clinical trial, which is expected to start in 2018. A clinical trial protocol has been ~~completed. We are working with academic collaborators to refine this genomic methylation test.~~developed and will be reviewed by the Institutional Review Board.
While the NCI will conduct a trial for RASopathy related cancers in pediatric patients, Onconova will focus on initiating a trial as well in Juvenile Myelomonocytic Leukemia (JMML), a well-described RASopathy affecting children which is incurable without an allogenic hematopoietic stem cell transplant.
Other Programs

The vast majority of the ~~Company's~~Company’s efforts are now devoted to the advanced stage development of rigosertib for unmet medical needs of MDS patients. Other programs are either paused, inactive or require only minimal internal resources and efforts.
Briciclib

Briciclib, another of our product candidates, is a small molecule targeting an important intracellular regulatory protein, ~~cyclin~~Cyclin D1, which is often found at elevated levels in cancer cells. Cyclin D1 expression is regulated through a process termed cap-dependent translation, which requires the function of eukaryotic initiation factor 4E ~~protein, or eIF4E.~~protein. In vitro evidence indicates briciclib binds

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to ~~eIF4E,~~eukaryotic initiation factor 4E protein, blocking cap-dependent translation of ~~cyclin~~Cyclin D1 and other cancer proteins, such as c-MYC, leading to tumor cell death. We have been conducting a Phase 1 ~~multisite~~multi-site dose-escalation trial of briciclib in patients with advanced solid tumors refractory to current therapies. Safety and efficacy assessments are complete in six of the seven dose-escalation cohorts of patients in this trial. As of December 2015, the Investigational New Drug (“IND”) for briciclib ~~IND~~ is on full clinical hold following a drug product lot testing failure. ~~If we elect to continue development of briciclib, we would~~We will be required to undertake appropriate remedial actions prior to re-initiating the clinical trial and completing the final dose-escalation cohort.
Recilisib
��
Recilisib is a product candidate being developed in collaboration with the U.S. Department of Defense for acute radiation syndromes. We have completed four Phase 1 trials to evaluate the safety and pharmacokinetics of recilisib in healthy human adult subjects using both subcutaneous and oral formulations. We have also conducted animal studies and clinical trials of recilisib under the ~~FDA's~~FDA’s Animal ~~Efficacy~~ Rule, which permits marketing approval for new medical countermeasures for which conventional human efficacy studies are not feasible or ethical, by relying on evidence from adequate and well-controlled studies in appropriate animal models to support efficacy in ~~humans.~~humans when the results of those studies establish that the drug is reasonably likely to produce a human clinical benefit. Human safety data, however, is still required. Ongoing studies of recilisib, focusing on animal models and biomarker development to assess the efficacy of recilisib are being conducted by third parties with government funding. We anticipate that any future development of recilisib beyond these ongoing studies would be conducted solely with government funding or by collaboration. Use of government funds to finance the research and development in whole or in part means any future effort to commercialize recilisib will be subject to federal laws and regulations on U.S. government rights in intellectual property. Additionally, we are subject to laws and regulations governing any research contracts, grants, or cooperative agreements under which government funding was provided.
Preclinical Product Candidates

In addition to our three clinical-stage product candidates, we have several product candidates that target kinases, cellular metabolism or cell division in preclinical development. We may explore additional collaborations to further the development of these product candidates as we focus internally on our more advanced programs. ~~We presented~~
Positive preclinical data ~~related to ON 0123300, our novel inhibitor or ARK5 and CDK4/6~~was announced at the American Association offor Cancer Research (AACR) ~~conference~~annual meeting, which took place April 1-5, 2017 in ~~April, 2016.~~
~~Recent Developments~~Washington, DC, for ON 123300, a first-in-class dual inhibitor of CDK4/6 + ARK5, and for ON 150030, a novel Type 1 inhibitor of FLT3 and Src pathways. We believe our CDK inhibitor is differentiated from other agents in the market (Palbociclib, Ribociclib and Abemaciclig) or in development (such as the compounds being developed by G1 Therapeutics) by its dual inhibition of CDK4/6 + ARK5. We are party to a license and collaboration agreement with HanX Biopharmaceuticals, Inc. (“HanX”), which grants HanX certain rights to commercialize ON 123300 in China. We continue to carry out research to enhance the pre-clinical data package for this compound in an attempt to seek additional partners outside of China for co-development of this novel compound.
~~We effected~~
In a ~~one-for-ten reverse stock split~~preclinical Rb+ve xenograft model for breast cancer, ON 123300 activity was shown to be similar to Palbociclib (Pfizer’s Ibrance®). Moreover, based on the same preclinical model, ON 123300 may have the potential advantage of ~~our common stock as of 5:00 PM Eastern Time~~reduced neutropenia when compared to Palbociclib. Whereas both compounds resulted in decreased RBC and platelet counts in this preclinical model system, Palbociclib was found to have a more prominent and statistically significant (P< 0.05) inhibitory effect on ~~May 31, 2016. All share and per share data has been restated~~neutrophil counts when compared to ~~reflect the one-for-ten reverse stock split, subject to final adjustments for treatment of fractional share interests.~~
~~Corporate Information~~ON 123300.

In December 2017, we entered into a license and collaboration agreement with HanX Biopharmaceuticals, Inc. (“HanX”), a company focused on development of novel oncology products, for the further development, registration and commercialization of ON 123300 in Greater China. Under the terms of the agreement, we received an upfront payment, and would receive regulatory and commercial milestone payments, as well as royalties on sales in the Greater China territory. The key feature of the collaboration is that HanX will provide all funding required for Chinese IND enabling studies necessary for filing an IND with the Chinese Food and Drug Administration. The studies would be conducted to meet the Good Laboratory Practice (“GLP”) requirements of FDA such that we could simultaneously file an IND with the US FDA. We and HanX will oversee the IND enabling studies. We will maintain global rights outside of China.
In March 2018, Onconova and HanX completed the pre-Investigational New Drug, or pre-IND, consultation with FDA. These discussions provided guidance for the manufacturing of ON 123300 and the pre-clinical development plan for the submission of an IND application.
In April 2018, at the American Association for Cancer Research 2018 Annual Meeting, we announced an advance in pre-clinical development and the presentation of new pre-clinical data for ON 123300. The data from preclinical studies demonstrates that there is a differential metabolism of ON 123300 in male versus female rodents. As a result, the drug exposure is almost 2-3 fold higher in female rats. Based upon preclinical animal liver microsome studies, this differential effect appears to be limited to rodents, and is not observed in preclinical studies with human liver microsomes. Based on the preclinical liver microsome metabolism data from other species, relevant species have been selected along with the dosing strategy to be implemented in GLP toxicological studies to be conducted by HanX.
CORPORATE INFORMATION
We were incorporated in Delaware in December 1998 and commenced operations in January 1999. Our principal executive offices are located at 375 Pheasant Run, Newtown, Pennsylvania 18940, and our telephone number is (267) 759-3680. Our website address is www.onconova.com. The information on, or that can be accessed through, our website is not part of this prospectus.
~~Implications of Being an Emerging Growth Company~~THE OFFERING
We qualify as an "emerging growth company" as defined in the Jumpstart our Business Startups Act of 2012, or the JOBS Act. An emerging growth company may take advantage of specified reduced reporting requirements and is relieved of certain other significant requirements that are otherwise generally applicable to public companies. As an emerging growth company,
•
~~we may present only two years of audited financial statements and only two years of related Management's Discussion & Analysis of Financial Condition and Results of Operations;~~

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•
~~we are exempt from the requirement to obtain an attestation and report from our auditors on the assessment of our internal control over financial reporting pursuant to the Sarbanes-Oxley Act of 2002;~~

•
~~we are permitted to provide less extensive disclosure about our executive compensation arrangements;~~

•
~~we are not required to give our stockholders non-binding advisory votes on executive compensation or golden parachute arrangements; and~~

•
~~we have elected to use an extended transition period for complying with new or revised accounting standards.~~
        We may take advantage of these provisions until July 24, 2018. However, if certain events occur prior to such date, including if we become a "large accelerated filer," our annual gross revenues exceed $1.0 billion or we issue more than $1.0 billion of non-convertible debt in any three-year period, we will cease to be an emerging growth company.

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~~SUMMARY CONSOLIDATED FINANCIAL DATA~~
        The following summary consolidated statements of operations data for the years ended December 31, 2015 and 2014 and the consolidated balance sheet data as of December 31, 2015, have been derived from our audited consolidated financial statements that are included in the documents incorporated by reference into this prospectus. The summary historical financial information as of and for the three months ended March 31, 2016 has been derived from our unaudited consolidated financial statements that are included in the documents incorporated by reference into this prospectus. The unaudited financial information as of and for the three months ended March 31, 2016 has been prepared on the same basis as our audited financial statements and includes, in the opinion of management, all adjustments, consisting of only normal recurring adjustments, necessary to fairly present the data for such periods. All share and per share data has been restated to reflect our one-for-ten reverse stock split effective May 31, 2016, subject to final adjustments for treatment of fractional share interests. The historical financial data presented below is not necessarily indicative of our financial results in future periods. You should read the summary consolidated financial data in conjunction with those financial statements and the accompanying notes and "Management's Discussion and Analysis of Financial Condition and Results of Operations" in the documents incorporated by reference into this prospectus. Our consolidated financial statements are prepared and presented in accordance with United States generally accepted accounting principles, or U.S. GAAP.
~~Condensed Consolidated Statements of Operations Data:~~

For the Years ended
December 31,

For the three
months ended
March 31, 2016

2015 2014
Revenue
$ 11,456,000 $ 800,000 $ 1,474,000
Operating expenses:

General and administrative
9,533,000 15,119,000 3,172,000
Research and development
25,895,000 49,425,000 5,822,000

Total operating expenses
35,428,000 64,544,000 8,994,000

Loss from operations
(23,972,000 ) (63,744,000 ) (7,520,000 )
Change in fair value of warrant liability

—
20,000
271,000
Other income (expense), net
(35,000 ) (52,000 ) 9,000

Net loss before income taxes
(24,007,000 ) (63,776,000 ) (7,240,000 )
Income taxes
16,000 19,000 —

Net loss
(24,023,000 ) (63,795,000 ) (7,240,000 )
Net loss attributable to non-controlling interest
44,000 113,000 —

Net loss attributable to Onconova Therapeutics, Inc
(23,979,000 ) (63,682,000 ) $ (7,240,000 )

Net loss per share of common stock, basic and diluted
$ (10.54 ) $ (29.41 ) $ (2.65 )

Basic and diluted weighted average shares outstanding
2,273,976 2,165,354 2,731,590

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~~Consolidated Balance Sheet Data:~~
~~(Stockholders'sequity numbers have been restated to reflect the one-for-ten~~
~~reverse stock split and decrease in authorized common shares from~~
~~75,000,000 to 25,000,000)~~

December 31,
2015 March 31, 2016
Assets

Current assets:

Cash and cash equivalents
$ 19,799,000 $ 16,835,000
Receivables
1,504,000 1,368,000
Prepaid expenses and other current assets
1,832,000 1,153,000
Restricted cash
50,000 50,000

Total current assets
23,185,000 19,406,000
Property and equipment, net
248,000 224,000
Other non-current assets
12,000 12,000

Total assets
$ 23,445,000 $ 19,642,000

Liabilities and stockholders' equity

Current liabilities:

Accounts payable
$ 3,421,000 $ 3,296,000
Accrued expenses and other current liabilities
3,729,000 3,891,000
Deferred revenue
455,000 455,000

Total current liabilities
7,605,000 7,642,000
Warrant liability
— 295,000
Deferred revenue, non-current
5,000,000 4,886,000

Total liabilities
12,605,000 12,823,000

Commitments and contingencies

Stockholders' equity:

Preferred stock, $0.01 par value, 5,000,000 authorized at March 31, 2016 and December 31, 2015, none issued and outstanding at March 31, 2016 and December 31, 2015
— —
Common stock, $0.01 par value, 25,000,000 authorized at March 31, 2016 and December 31, 2015, 2,740,103 and 2,546,419 shares issued
25,000 27,000
Additional paid-in capital
328,564,000 331,775,000
Accumulated other comprehensive loss
(22,000 ) (16,000 )
Accumulated deficit
(318,557,000 ) (325,797,000 )

Total Onconova Therapeutics, Inc. stockholders' equity
10,010,000 5,989,000
Non-controlling interest
830,000 830,000

Total stockholders' equity
10,840,000 6,819,000

Total liabilities and stockholders' equity
$ 23,445,000 $ 19,642,000

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~~SUMMARY OF THE RIGHTS OFFERING~~
Units offered by us in this offering
Up to 26,785,714 Units, each consisting of one share of Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock.
~~Securities to be offered~~
Pre-Funded Units offered by us in this offering
We are ~~distributing~~also offering to ~~you, at no charge, one non-transferable Subscription Right~~each purchaser whose purchase of Units in this offering would otherwise result in the purchaser, together with its affiliates and certain related parties, beneficially owning more than 4.99% of our outstanding Common Stock immediately following the consummation of this offering, the opportunity to purchase, if the purchaser so chooses, Pre-Funded Units (each Pre-Funded Unit consisting of one Unit for every share of our common stock that you owned on the Record Date, either as a holder of record or, in the case of shares held of record by brokers, banks, or other nominees, on your behalf, as a beneficial owner of such shares. Each Unit consists of            share of common stock andPre-Funded Warrant ~~representing the right~~ to purchase one share of ~~common stock~~Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock) in lieu of Units that would otherwise result in the purchaser’s beneficial ownership exceeding 4.99% of our outstanding Common Stock (or at the election of the purchaser, 9.99%). Each Pre-Funded Warrant contained in a Pre-Funded Unit will be exercisable for one share of Common Stock. The purchase price of each Pre-Funded Unit will equal the price per Unit being sold to the public in this offering minus $0.01, and the exercise price of each Pre-Funded Warrant included in the Pre-Funded Unit will be $0.01 per share of Common Stock. This offering also relates to the shares of Common Stock issuable upon exercise of any Pre-Funded Warrants contained in the Pre-Funded Units sold in this offering. For each Pre-Funded Unit we sell, the number of Units we are offering will be decreased on a one-for-one basis. Because we will issue a Warrant as part of each Unit or Pre-Funded Unit, the number of Warrants sold in this offering will not change as a result of a change in the mix of the Units and Pre-Funded Units sold.
Warrants offered by us in this offering
Warrants to purchase an aggregate of up to 669,643 shares of Series B Preferred Stock. Each Unit and each Pre-Funded Unit includes a Warrant to purchase 0.025 share of Series B Preferred Stock. Each Warrant contained in a Unit or Pre-Funded Unit has an exercise price of $         per ~~share.~~
~~Holders of our currently outstanding participating warrants will also be entitled, pursuant to the terms of such warrants, to receive one Subscription Right to purchase one Unit for every~~0.025 share of ~~common stock~~Series B Preferred Stock, will be immediately separable from the Common Stock or Pre-Funded Warrant, as ~~if each~~the case may be, will be exercisable immediately and will expire on the 18-month anniversary of ~~such participating warrants had been exercised immediately prior to~~ the ~~record date for the Rights Offering.~~Charter Amendment Date.
~~Size~~
Series B Preferred Stock
This prospectus also relates to the offering of ~~offering~~
            ~~Units,~~the shares of Series B Preferred Stock issuable upon the exercise of the Warrants. The Series B Preferred Stock is not convertible until the next business day after the Charter Amendment Date, starting at which time each ~~consisting of~~0.025 share of ~~common stock and            Warrant representing~~ the ~~right to purchase~~Series B Preferred Stock is convertible into one share of ~~common stock~~Common Stock. Notwithstanding the foregoing, we shall not effect any conversion of the Series B Preferred Stock, to the extent that, after giving effect to an attempted conversion, the holder of shares of Series B Preferred Stock (together with such holder’s affiliates, and any persons acting as a group together with such holder or any of such holder’s affiliates) would beneficially own a number of shares of Common Stock in excess of 4.99% of the shares of Common Stock then outstanding after giving effect to such exercise. In the event our stockholders do not approve the Charter Amendment, the Series B Preferred Stock will not be convertible into Common Stock and the value of the Warrants and the Series B Preferred Stock may be negatively affected. For additional information, see the subsection entitled “Description of Securities We Are Offering —Series B Convertible Preferred Stock” in this prospectus.
Insider Participation
One or more of our directors have indicated interests in purchasing approximately 10% of the Units to be sold in this offering at the public offering price and on the same terms as the other purchasers in this offering. However, because indications of interest are not binding agreements or commitments to purchase, the underwriter could determine to sell more, fewer or no Units to our director(s) in this offering, or our director(s) could determine to purchase more, fewer or no Units in this offering.
Offering Price
$          per Unit
$         per Pre-Funded Unit
Option to purchase additional securities
The underwriter has the option to purchase up to 4,017,857 additional shares of Common Stock at a purchase price of $       per share and/or Warrants to purchase up to an aggregate of 100,446 shares of Series B Preferred Stock at a purchase price of $0.01 per Warrant with an exercise price of $           per ~~share.~~
~~Subscription Price~~
~~$            per Unit.~~
~~Warrants~~
~~Each Warrant entitles the holder to purchase one~~0.025 share of ~~common stock~~Series B Preferred Stock, less the underwriting discounts and commissions. The underwriter may exercise its option at ~~an exercise price of $            per share~~any time and from time to time within 30 days from the date of issuance through its expiration on                    . The Warrants will be exercisable by paying the exercise price in cash, or, solely during any period when a registration statement for the exercise of the Warrants is not in effect, exercisable on a cashless basis. We intend to apply to list the Warrants on NASDAQ under the symbol "ONTXW," although there is no assurance that a sufficient number of Subscription Rights will be exercised so that the Warrants will meet the minimum listing criteria to be accepted for listing on NASDAQ. We may redeem the Warrants for $            per Warrant if our common stock closes above $            per share for 10 consecutive trading days.this prospectus.
~~Record Date~~
~~5:00 PM Eastern Time,                     , 2016.~~
~~Subscription Right~~Common stock to be outstanding after this offering
~~Each Subscription Right consists~~46,211,877 shares of ~~a Basic Subscription Right and an Over-Subscription Privilege.~~Common Stock (assuming no sale of any Pre-Funded Units), or 50,229,734 shares of Common Stock if the underwriter exercises its option to purchase additional Units in full (assuming no sale of any
Pre-Funded Units).
~~Basic Subscription Rights~~
~~Basic Subscription Right will entitle you to purchase one Unit at the Subscription Price.~~

~~Table of Contents~~
~~Over-Subscription Privilege~~
If you exercise your Basic Subscription Rights in full, you may also choose to purchase a portion of any Units that are not purchased by our other shareholders or holders of participating warrants through the exercise of their Basic Subscription Rights. You may subscribe for additional Units pursuant to this Over-Subscription Privilege, subject to proration described elsewhere.
~~Expiration date~~Use of proceeds
~~The Subscription Rights will expire at 5:00 PM Eastern Time, on                    , 2016.~~ We ~~reserve~~estimate that the ~~right~~net proceeds to ~~extend the expiration date in our sole discretion.~~
~~Procedure for exercising Subscription Rights~~
~~To exercise your Subscription Rights, you must take the following steps:~~
If you are a record holder of our common stock or a holder of participating warrants, you must deliver payment and a properly completed Subscription Rights Statement to the Subscription Agent to be received before 5:00 PM Eastern Time, on                    , 2016. You may deliver the documents and payments by first class mail or courier service. If you use first class mail forus from this ~~purpose, we recommend using registered mail, properly insured, with return receipt requested.~~
If you are a beneficial owner of shares that are registered in the name of a broker, dealer, custodian bank, or other nominee, you should instruct your broker, dealer, custodian bank, or other nominee to exercise your Subscription Rights on your behalf. Please follow the instructions of your nominee, who may require that you meet a deadline earlier than 5:00 PM Eastern Time, on                    , 2016.
~~Delivery of shares and Warrants~~
As soon as practicable after the expiration of the Rights Offering, the Subscription Agent will arrange for the issuance of the shares of common stock and Warrants purchased pursuant to the Rights Offering. All shares and Warrants that are purchased in the Rights Offeringoffering will be ~~issued~~approximately $13.4 million ($15.4 million if the underwriter’s option to purchase additional Units is exercised in book-entry, or uncertificated, form meaning that you will receive a direct registration (DRS) account statement from our transfer agent reflecting ownership of these securities if you are a holder of record of shares or warrants. If you hold your shares in the name of a custodian bank, broker, dealer, or other nominee, DTC will credit your account with your nominee with the securities you purchased in the Rights Offering.
~~Non-transferability of Subscription Rights~~
~~The Subscription Rights may not be sold, transferred, assigned or given away to anyone. The Subscription Rights will not be listed for trading on any stock exchange or market.~~

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~~Transferability of Warrants~~
~~The Warrants will be separately transferable following their issuance and through their expiration            years from the issuance date.~~
~~No board recommendation~~
~~Our board of directors is not making a recommendation regarding your exercise of the Subscription Rights. You are urged to make your decision to invest~~full), based on ~~your own assessment~~an assumed public offering price per Unit of ~~our business~~$0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018, and ~~the Rights Offering. Please see "Risk Factors" for a discussion of some of the risks involved in investing in our securities.~~
~~No revocation~~
~~All exercises of Subscription Rights are irrevocable, even if you later learn of information that you consider to be unfavorable to the exercise of your Subscription Rights.~~
~~Use of proceeds~~
after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us. We intend to use the net proceeds from this ~~Rights Offering~~offering to ~~continue funding INSPIRE, our ongoing Phase 3 clinical trial of rigosertib IV in a population of patients with higher-risk MDS after failure of HMA therapy, and, to a lesser extent, for other~~fund the development of our clinical and preclinical programs, for other research and development activities ~~business development~~ and for general corporate purposes, which may include capital expenditures and funding ~~our~~ working capital needs. See ~~"Use~~“Use of ~~Proceeds."~~Proceeds” on page 16.
~~Material U.S. federal income tax consequences~~
For U.S. federal income tax purposes, we do not believe you should recognize income or loss upon receipt or exercise of a Subscription Right. You should consult your own tax advisor as to the tax consequences of the Rights Offering in light of your particular circumstances. See "Material U.S. Federal Income Tax Consequences."
~~Extension and termination~~Risk factors
~~Although we do not presently intend to do so, we may extend~~You should read the ~~Rights Offering for additional time in our sole discretion. Our board~~“Risk Factors” section of ~~directors may for any reason terminate the Rights Offering at any time before the completion of the Rights Offering.~~
~~Subscription Agent~~
~~Wells Fargo Bank, N.A.~~
~~Questions~~
If you have any questions about the Rights Offering, please contact the dealer-manager, Maxim Group LLC, at 405 Lexington Avenue, New York, New York 10174, Attention Syndicate Department, email: syndicate@maximgrp.com or telephone: (212) 895-3745.
~~Market for common stock~~
~~Our common stock is listed on NASDAQ under the symbol "ONTX." See "Market Price of our Common Stock~~this prospectus and ~~Related Stockholder Matters."~~

~~Table of Contents~~
~~Risk factors~~
~~Before you exercise your Subscription Rights to purchase Units, you should be aware that there are risks associated with your investment, and you should carefully read and consider risks described~~ in the ~~section captioned "Risk Factors" together with all of the other information included in~~documents incorporated by reference into this ~~prospectus.~~
~~Dealer-Manager~~
~~Maxim Group LLC.~~
~~Distribution arrangements~~
Under the terms and subject to the conditions contained in the dealer-manager agreement, the dealer-manager will use its best efforts to solicit the exercise of Subscription Rights. We have agreed to pay the dealer-manager certain fees for acting as dealer-manager and to reimburse the dealer-manager for certain out-of-pocket expenses incurred in connection with this offering. The dealer-manager is not underwriting or placing any of the Subscription Rights or the Units, shares of common stock or Warrants being issued in this offering, and do not make any recommendation with respect to such Subscription Rights (including with respect to the exercise or expiration of such Subscription Rights), Units, shares of common stock or Warrants. See "Plan of Distribution"prospectus for a discussion of factors to consider before deciding to invest in our securities.
Listing
Common Stock is listed on the ~~fees~~Nasdaq Capital Market under the symbol “ONTX.” We do not intend to apply for listing of the Pre-Funded Warrants, the Warrants or Series B Preferred Stock on any securities exchange or other nationally recognized trading system. There is no established public trading market for the Pre-Funded Warrants, the Warrants or Series B Preferred Stock, and ~~expenses~~we do not expect a market to ~~be paid to the dealer-manager.~~develop.

The number of shares of Common Stock outstanding after the offering is based on 19,426,163 shares outstanding as of March 31, 2018, and excludes as of such date:
~~Table~~·                  1,118,849 shares of ~~Contents~~Common Stock issuable upon the exercise of stock options outstanding at March 31, 2018 with a weighted average exercise price of approximately $25.00 per share;
·                  15,232,146 shares of Common Stock issuable upon the exercise of outstanding or issuable warrants at March 31, 2018 with a weighted average exercise price of approximately $1.82 per share (includes Common Stock issuable for warrants which are exercisable for our Series A Convertible Preferred Stock, which is convertible to Common Stock); to the extent the preferred stock warrant repricing described under “Underwriting – Company Lock-up Waiver Agreement” occurs, preferred stock warrants to purchase an aggregate of 994,750 shares of our Series A Convertible Preferred Stock, which are convertible into an aggregate of 9,947,500 shares of Common Stock, will be repriced, and the weighted average exercise price of all outstanding or issuable warrants at March 31, 2018 taking into account such repricing will be approximately $1.55 per share based on an assumed public offering price per Unit of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018;
·                  0 shares of Common Stock issuable upon the conversion of our Series A Convertible Preferred Stock at March 31, 2018 (no shares of Series A Preferred Stock were issued and outstanding and 1,044,488 shares of Series A Preferred Stock were reserved for issuance upon the exercise of outstanding preferred stock warrants as of March 31, 2018);
·                  33,779 shares of Common Stock reserved for future issuance under our 2013 Equity Compensation Plan at March 31, 2018;
·                  any additional shares of Common Stock that we may issue to Lincoln Park Capital Fund, LLC (“Lincoln Park”), pursuant to a purchase agreement we entered into on October 8, 2015, which provides that, upon the terms and subject to the conditions and limitation set forth therein, Lincoln Park is committed to purchase up to an aggregate of an additional $15 million of shares of Common Stock over the term of the purchase agreement, should we elect to sell shares to Lincoln Park; and
·      shares of Common Stock issuable upon the exercise of the Underwriter’s Warrants issued in this offering.
As of April 20, 2018, the total number of our outstanding shares of Common Stock was 20,243,108.
Unless otherwise indicated, all information contained in this prospectus assumes (i) no exercises by the underwriter of its option to purchase additional securities and (ii) no sale of any Pre-Funded Warrants.

RISK FACTORS

Our business is influenced by many factors that are difficult to predict, and that involve uncertainties that may materially affect actual operating results, cash flows and financial condition. Before making an investment decision, you should carefully consider these risks, including those set forth below and those described in the ~~"Risk Factors"~~“Risk Factors” section of our ~~most recent~~ Annual Report on Form 10-K, ~~filed with the Commission on March 28, 2016,~~ as ~~revised or supplemented by our Quarterly Reports on Form 10-Q~~ filed with the SEC ~~since the filing of our most recent Annual Report~~ on ~~Form 10-K, each of~~March 16, 2018, which is incorporated by reference into this prospectus, as well as any amendment or update to our risk factors reflected in subsequent filings with the SEC, and you should also carefully consider any other information we include or incorporate by reference in this prospectus.

Any of the risks we describe below or in the information incorporated herein by reference in this prospectus could cause our business, financial condition or operating results to suffer. The market price of ~~our common stock~~Common Stock could decline if one or more of these risks and uncertainties develop into actual events. You could lose all or part of your investment.
Risks ~~Related~~Associated with this Offering
Our management will have broad discretion over the use of any net proceeds from this offering, you may not agree with how we use the proceeds, and the proceeds may not be invested successfully.
Our management will have broad discretion as to the ~~Rights Offering~~
Weuse of any net proceeds from this offering and could use them for purposes other than those contemplated at the time of this offering. Accordingly, you will ~~incur substantial expenses in connection~~be relying on the judgment of our management with ~~the Rights Offering, which may not return adequate value if the Rights Offering is ultimately not consummated or successful.~~
        The estimated expenses for the Rights Offering are approximately $            , excluding fees and expenses of the dealer-manager that we have engaged to assist us with the Rights Offering. If the registration statement of which this prospectus is a part is not declared effective, the Rights Offering is not commenced or the Rights Offering is not ultimately consummated or successful, we will incur these expenses nonetheless. We have providedregard to the ~~dealer-manager a non-accountable expense allowance~~use of ~~$100,000 for expenses incurred in connection with~~any proceeds from the ~~Rights Offering, and we advanced $30,000~~sale of ~~this $100,000 allowance to Maxim Group LLC upon its engagement as a dealer-manager. If its engagement is terminated, Maxim Group LLC will promptly reimburse to us any portion~~shares of ~~the advance not used for actual out-of-pocket expenses in connection with the Rights Offering. See "Plan of Distribution."~~
~~Your interest in~~ our ~~company may be diluted as a result of this Rights Offering.~~
        Shareholders who do not fully exercise their Subscription Rights should expect that they will, at the completion of this offering, own a smaller proportional interest in our company than would otherwise be the case had they fully exercised their Basic Subscription Right and Over-Subscription Privilege. Further, the shares issuable upon the exercise of the Warrants to be issued pursuant to the Rights Offering will dilute the ownership interest of shareholders not participatingsecurities in this offering, and you will not have the opportunity, as part of your investment decision, to assess whether the proceeds are being used appropriately. It is possible that the proceeds will be invested in a way that does not yield a favorable, or any, return for you.
We may be required to raise additional financing by issuing new securities with terms or rights superior to those of our existing securityholders, which could adversely affect the market price of shares of Common Stock and our business.
We will require additional financing to fund future operations, including expansion in current and new markets, development and acquisition, capital costs and the costs of any necessary implementation of technological innovations or alternative technologies. We may not be able to obtain financing on favorable terms, if at all. If we raise additional funds by issuing equity securities, the percentage ownership of our current stockholders will be reduced, and the holders of ~~Warrants issued pursuant~~the new equity securities may have rights superior to ~~this offering who~~those of our existing securityholders, which could adversely affect the market price of Common Stock and the voting power of shares of Common Stock. If we raise additional funds by issuing debt securities, the holders of these debt securities would similarly have ~~not exercised them.~~some rights senior to those of our existing securityholders, and the terms of these debt securities could impose restrictions on operations and create a significant interest expense for us which could have a materially adverse effect on our business.
~~Further, because the price per Unit being offered may be substantially higher than~~
You will experience immediate and substantial dilution in the net tangible book value per share of ~~our common stock, you may suffer substantial dilution~~Common Stock included in the Units or issuable upon exercise of the Pre-Funded Warrants in this offering.
Since the effective price per share of Common Stock included in the Units or issuable upon exercise of the Pre-Funded Warrants being offered is substantially higher than the net tangible book ~~value~~deficit per share of ~~the common stock you purchase in this offering. If you purchase Units in~~Common Stock outstanding prior to this offering, ~~at the Subscription Price,~~ you ~~may~~will suffer immediate and substantial dilution in the net tangible book value of Common Stock included in the ~~common stock. See "Dilution"~~Units or issuable upon the exercise of the Pre-Funded Warrants issued in this ~~prospectus~~offering. See the section titled “Dilution” below for a more detailed discussion of the dilution ~~which may~~you will incur ~~in connection with this offering.~~
Completion of the Rights Offering is not subject to us raising a minimum offering amount and we will still need additional funding to carry out our proposed operating activities, including our INSPIRE trial, after the Rights Offering.
        Completion of the Rights Offering is not subject to us raising a minimum offering amount and therefore the net proceeds from the Rights Offering may be insufficient to meet our objectives, thereby increasing the risk to investorsif you purchase Units in this ~~offering, including investing in a company that continues~~offering. To the extent outstanding stock options or warrants to

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~~require capital. Even if we sell all of the Units subject~~ purchase Common Stock are exercised, or outstanding warrants to ~~the Rights Offering, we will need to obtain additional financing in the future in order to fully fund rigosertib or any other product candidates through the regulatory approval process.~~
        ~~Due to our ongoing losses and our accumulated deficit in combination with other factors, the opinion~~purchase shares of our independent registered public accounting firm on our audited consolidated financial statements for our fiscal year ended December 31, 2015 contains an explanatory paragraph regarding substantial doubt about our ability to continue as a going concern. Even if we sell all of the Units subject to the Rights Offering, if we are unable to obtain sufficient additional funding, through future financings or through strategic and collaborative transactions and arrangements, we may not have sufficient resources to complete our INSPIRE trial and we may continue to delay, scale-back or eliminate certain of our planned research, drug discovery and development activities and certain other aspects of our operations, or we may not be able to continue as a going concern.
~~This Rights Offering may cause the trading price of our common stock to decrease.~~
        The Subscription Price, together with the number of shares of common stock we propose to issue and ultimately will issue if this Rights Offering is completed, may result in an immediate decrease in the market price of our common stock. This decrease may continue after the completion of this Rights Offering. If that occurs, you may have committed to buy shares of common stock in the Rights Offering at a price greater than the prevailing market price. We cannot predict the effect, if any, that the availability of shares for future sale represented by the Warrants issued in connection with the Rights Offering, or the ability to trade the Warrants themselves, will have on the market price of our common stock from time to time. Further, if a substantial number of Subscription RightsSeries A Convertible Preferred Stock are exercised and the ~~holders~~resulting shares of Series A Convertible Preferred Stock are converted into shares of Common Stock, there will be further dilution to new investors.
Our shareholders may experience significant dilution as a result of future equity offerings or issuances.
In order to raise additional capital or pursue strategic transactions, we may in the future offer, issue or sell additional shares ~~received upon exercise~~ of ~~those Subscription Rights~~Common Stock or ~~the related Warrants choose to sell some~~other securities convertible into or ~~all~~exchangeable for shares of ~~the shares underlying the Subscription Rights or the related Warrants, the resulting sales could depress the market price of our common stock. Following the exercise of your Subscription Rights~~Common Stock. We cannot assure you ~~may not~~that we will be able to sell ~~your common stock~~shares or other securities in any other transaction at a price per share or that have an exercise price or conversion price per shares that is equal to or greater than the ~~Subscription Price.~~
~~Because~~price for the ~~exercise of your Subscription Rights is not revocable, you could be committed to buying shares of common stock above the prevailing market price.~~
        Once you exercise your Subscription Rights, you may not revoke such exercise even if you later learn information that you consider to be unfavorable to the exercise of your Subscription Rights. The market price of our shares of common stock may decline prior to the expiration of this offering or a Subscribing Rights holder may not be able to sell shares of common stocksecurities purchased by investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at ~~a price equal to~~which we sell or ~~greater than the Subscription Price. Until~~issue additional shares of ~~our common stock are delivered upon expiration of the Rights Offering, you will not be able to sell~~Common Stock or ~~transfer the shares of our common stock that you purchase in the Rights Offering. Any such delivery will occur as soon as practicable after the Rights Offering has expired, payment~~other securities convertible into or exchangeable for ~~the shares of common stock and attached Warrants subscribed for has cleared, and all prorating calculations and reductions contemplated by the terms of the Rights Offering have been effected.~~
~~If we terminate this offering for any reason, we will have no obligation other than to return subscription monies as soon as practicable.~~
        We may decide, in our sole discretion and for any reason, to cancel or terminate the Rights Offering at any time prior to the expiration date. If this offering is cancelled or terminated, we will have no obligation with respect to Subscription Rights that have been exercised except to return as soon as practicable, without interest, the subscription payments deposited with the Subscription Agent. If we terminate this offering and you have not exercised any Subscription Rights, such Subscription Rights will expire worthless.

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~~Our common stock price may be volatile as a result of this Rights Offering.~~
        ~~The trading price of our common stock may fluctuate substantially. The price of the common stock that will prevail in the market after this offering~~Common Stock future transactions may be higher or lower than the Subscription Price depending on many factors, some of which are beyond our control and may not be directly related to our operating performance. These factors include, but are not limited to, the following:
•
~~results of clinical trials of our product candidates or those of our competitors;~~

•
~~regulatory actions with respect to our products or our competitors' products;~~

•
~~actual or anticipated changes in our growth rate relative to our competitors;~~

•
~~announcements by us or our competitors of significant acquisitions, strategic partnerships, joint ventures, collaborations or capital commitments;~~

•
~~the success of competitive products or technologies;~~

•
~~regulatory or legal developments in the United States and other countries;~~

•
~~developments or disputes concerning patent applications, issued patents or other proprietary rights;~~

•
~~the recruitment or departure of key personnel;~~

•
~~the level of expenses related to any of our product candidates or clinical development programs;~~

•
~~the results of our efforts to in-license or acquire additional product candidates or products;~~

•
~~actual or anticipated changes in estimates as to financial results, development timelines or recommendations by securities analysts;~~

•
~~variations in our financial results or those of companies that are perceived to be similar to us;~~

•
~~fluctuations in the valuation of companies perceived by investors to be comparable to us;~~

•
~~share price and volume fluctuations attributable to inconsistent trading volume levels of our shares;~~

•
~~announcement or expectation of additional financing efforts;~~

•
~~sales of our common stock by us, our insiders or our other stockholders;~~

•
~~changes in the structure of healthcare payment systems;~~

•
~~market conditions in the pharmaceutical and biotechnology sectors; and~~

•
~~general economic, industry and market conditions.~~
        Additionally, the stock market historically has experienced significant price and volume fluctuations. These fluctuations are often unrelated to the operating performance of particular companies. These broad market fluctuations may cause declines in the trading price and market value of our common stock.such price.
We cannot assure you that the trading price of our common stock will not decline after you elect to exercise your Subscription Rights. If that occurs, you may have committed to buy shares of common stock in the Rights Offering at a price greater than the prevailing market price and could have an immediate unrealized loss. Moreover, we cannot assure you that, following the exercise of your Subscription Rights, you will be able to sell your common stock at a price equal to or greater than the Subscription Price, and you may lose all or part of your investment in our common stock. Until shares are delivered upon expiration of the Rights Offering, you will not be able to sell the shares of our common stock that you purchase in the Rights Offering. Shares of our common stock purchased will be

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issued as soon as practicable after the Rights Offering has expired, payment for the shares of common stock and attached Warrants subscribed for has cleared, and all prorating calculations and reductions contemplated by the terms of the Rights Offering have been effected. We will not pay you interest on funds delivered to the Subscription Agent pursuant to your exercise of Subscription Rights.
Because we do not have any formal commitments from any of our shareholders to participate in the Rights Offering, the net proceeds we receive from the Rights Offering may be lower than we currently anticipate.
        We do not have any formal commitments from any of our shareholders to participate in the Rights Offering, and we cannot assure you that any of our shareholders or warrant holders will exercise all or any part of their Basic Subscription Rights or their Over-Subscription Privilege. If our shareholders or participating warrant holders subscribe for fewer shares of our common stock than we currently anticipate, the net proceeds we receive from the Rights Offering could be significantly lower than we currently expect.
~~The Subscription Price determined for this offering is not an indication of the fair value of our common stock.~~
        In determining the Subscription Price, the pricing committee of our board of directors considered a number of factors, including, but not limited to, the price at which our stockholders might be willing to participate in the Rights Offering, the value of the Warrant being issued as a component of the Unit, historical and current trading prices for our common stock, the amount of proceeds desired, the potential need for liquidity and capital, potential market conditions, and the desire to provide an opportunity to our shareholders to participate in the Rights Offering. In conjunction with its review of these factors, our pricing committee also reviewed a range of discounts to market value represented by the subscription prices in various prior Rights Offerings by other public companies. The Subscription Price does not necessarily bear any relationship to the book value of our assets, results of operations, cash flows, losses, financial condition or any other established criteria for value. You should not consider the Subscription Price as an indication of the fair value of our common stock. After the date of this prospectus, our common stock may trade at prices above or below the Subscription Price.
~~If you do not act on a timely basis and follow subscription instructions, your exercise of Subscription Rights may be rejected.~~
        Holders of Subscription Rights who desire to purchase shares of our common stock and attached Warrants in this offering must act on a timely basis to ensure that all required forms and payments are actually received by the Subscription Agent prior to 5:00 PM Eastern Time, on the expiration date, unless extended. If you are a beneficial owner of shares of common stock and you wish to exercise your Subscription Rights, you must act promptly to ensure that your broker, dealer, custodian bank, trustee or other nominee acts for you and that all required forms and payments are actually received by your broker, dealer, custodian bank, trustee or other nominee in sufficient time to deliver such forms and payments to the Subscription Agent to exercise the Subscription Rights granted in this offering that you beneficially own prior to 5:00 PM Eastern Time on the expiration date, as may be extended. We will not be responsible if your broker, dealer, custodian bank, trustee or other nominee fails to ensure that all required forms and payments are actually received by the Subscription Agent prior to 5:00 PM Eastern Time, on the expiration date.
        If you fail to complete and sign the required subscription forms, send an incorrect payment amount, or otherwise fail to follow the subscription procedures that apply to your exercise in this Rights Offering, the Subscription Agent may, depending on the circumstances, reject your subscription or accept it only to the extent of the payment received. Neither we nor the Subscription Agent undertakes to contact you concerning an incomplete or incorrect subscription form or payment, nor are

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~~we under any obligation to correct such forms or payment. We have the sole discretion to determine whether a subscription exercise properly follows the subscription procedures.~~
~~You may not receive all of the Units for which you over-subscribe.~~
        Holders who fully exercise their Basic Subscription Rights will be entitled to subscribe for an additional number of Units. Over-Subscription Privileges will be allocated pro rata among Rights holders who over-subscribed, based on the number of over-subscription Units to which they have subscribed. We cannot guarantee that you will receive any or the entire amount of Units for which you over-subscribed. If the prorated amount of Units allocated to you in connection with your Over-Subscription Privilege is less than your Over-Subscription Request, then the excess funds held by the Subscription Agent on your behalf will be returned to you, without interest, as soon as practicable after the Rights Offering has expired and all prorating calculations and reductions contemplated by the terms of the Rights Offering have been effected, and we will have no further obligations to you.
~~The receipt of Subscription Rights may be treated as a taxable distribution to you.~~
        We believe the distribution of the Subscription Rights in this Rights Offering should be a non-taxable distribution to holders of shares of common stock and the participating warrants under Section 305(a) of the Internal Revenue Code of 1986, as amended, or the "Code." Please see the discussion on the "Material U.S. Federal Income Tax Consequences" below. This position is not binding on the IRS, or the courts, however. If this Rights Offering is deemed to be part of a "disproportionate distribution" under Section 305 of the Code, your receipt of Subscription Rights in this offering may be treated as the receipt of a taxable distribution to you equal to the fair market value of the Subscription Rights. Any such distribution would be treated as dividend income to the extent of our current and accumulated earnings and profits, if any, with any excess being treated as a return of capital to the extent thereof and then as capital gain. Each holder of shares of common stock and each holder of a participating warrant is urged to consult his, her or its own tax advisor with respect to the particular tax consequences of this Rights Offering.
~~The Subscription Rights are not transferable, and there~~There is no public market for the ~~Subscription Rights.~~Warrants, the Pre-Funded Warrants or the Series B Preferred Stock underlying the Warrants.
~~You may not sell, transfer, assign or give away your Subscription Rights. Because the Subscription Rights are non-transferable, there~~
There is no market or other means for you to directly realize any value associated with the Subscription Rights. You must exercise the Subscription Rights to realize any potential value from your Subscription Rights.
~~Absence of a~~established public trading market for the Warrants, ~~may limit your ability to resell~~ the ~~Warrants.~~
        ~~There is no established trading market for~~Pre-Funded Warrants or the Series B Preferred Stock underlying the Warrants, and we do not expect a market to ~~be issued pursuant to this offering, and the Warrants may~~develop. In addition, we do not ~~be widely distributed. We~~ intend to apply to list the Warrants, ~~for~~the Pre-Funded Warrants or the Series B Preferred Stock underlying the Warrants on any national securities exchange or other nationally recognized trading system, including The Nasdaq Capital Market. Without an active market, the liquidity of the Warrants, the Pre-Funded Warrants or the Series B Preferred Stock underlying the Warrants will be limited.
The Warrants and the Pre-Funded Warrants in this offering are speculative in nature.
Neither the Warrants nor the Pre-Funded Warrants in this offering confer any rights of Common Stock or Series B Preferred Stock ownership on ~~NASDAQ under~~its holders, such as voting rights or the ~~symbol "ONTXW,"~~right to receive dividends, but rather merely represent the right to acquire shares of Common Stock or Series B Preferred Stock at a fixed price, as the case maybe, and, with respect to the Warrants, during a fixed period of time. Specifically, commencing on the date of issuance, holders of the Warrants may exercise their right to acquire Series B Preferred Stock and pay an exercise price of $            per 0.025 share of Series B Preferred Stock, subject to certain adjustments, prior to the expiration of the Warrants.
Moreover, following this offering, the market value of the Warrants and the Pre-Funded Warrants, if any, is uncertain and there can be no assurance that the market value of the Warrants or the Pre-Funded Warrants will equal or exceed their imputed offering price. Neither the Warrants nor the Pre-Funded Warrants will be listed or quoted for trading on any market or exchange.
If we do not obtain shareholder approval to increase the number of our authorized shares of common stock in an amount sufficient to issue shares to those who purchase Warrants in this offering, the Warrants included in this offering may not have any value and you could lose part or all of your investment.
We do not currently have a sufficient number of ~~Subscription Rights~~authorized shares of Common Stock to cover the shares issuable upon conversion of the Series B Preferred stock being offered by this prospectus. As a result, before the Series B Preferred Stock can become convertible, we need to receive stockholder approval of the Charter Amendment (which is an amendment to our Tenth Amended and Restated Certificate of Incorporation, as amended, to sufficiently increase our authorized shares of Common Stock to cover the conversion of all outstanding shares of Series B Preferred Stock into Common Stock). We have agreed in the underwriting agreement for this offering to use our reasonable efforts to obtain such approval within 45 days from the date of this prospectus, and we intend to seek such approval at a special meeting of stockholders or at our 2018 annual meeting of stockholders. We cannot assure you that we will be ~~exercised so that the Warrants will meet minimum listing criteria~~able to ~~be accepted for listing on NASDAQ or that a market will develop for the Warrants. Even if a market for the Warrants does develop, the price~~obtain requisite stockholder approval of the ~~Warrants may fluctuate~~Charter Amendment. In the event our stockholders do not approve the Charter Amendment, the Series B Preferred Stock will not be convertible into Common Stock and ~~liquidity may be limited. If~~ the ~~Warrants are not accepted for listing on NASDAQ or if a market for the Warrants does not develop, then purchasers~~value of the Warrants may be unable to resell the Warrants or sell them only at an unfavorable price for an extended period of time, if at all. Future trading prices of the Warrants will depend on many factors, including:
•
~~our operating performance and financial condition;~~

•
~~our ability to continue the effectiveness of the registration statement, of which this prospectus is a part, covering~~ the Warrants and the ~~common stock issuable upon exercise~~Series B Preferred Stock may be negatively affected.
Sales of a significant number of shares of Common Stock in the ~~Warrants;~~

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•
public markets, or the ~~interest of securities dealers in making a market; and~~

•
~~the market for similar securities.~~
~~The market price of our common stock may never exceed the exercise price of the Warrants issued in connection with this offering.~~
        ~~The Warrants being issued in connection with this offering become exercisable upon issuance and will expire                years after issuance. We cannot provide you any assurance~~perception that such sales could occur, could depress the market price of Common Stock.
Sales of a substantial number of shares of Common Stock or securities convertible or exchangeable into Common Stock in the public markets could depress the market price of Common Stock and impair our ~~common stock~~ability to raise capital through the sale of additional equity securities. We cannot predict the effect that future sales of Common Stock would have on the market price of Common Stock.
Upon completion of this offering, based on our shares outstanding as of March 31, 2018, we will ~~ever exceed~~have 46,211,877 shares of Common Stock outstanding based on the issuance and sale of 26,785,714 Units in this offering, assuming no sale of any Pre-Funded Units. Of these shares, only 1,604,207 shares are subject to a contractual lock-up with the underwriter for this offering for a period of 90 days following this offering. These shares can be sold, subject to any applicable volume limitations under federal securities laws, after the earlier of the expiration of, or release from, the 90-day lock-up period. The balance of our outstanding shares of Common Stock, including any shares of Common Stock included in the Units, and shares of Common Stock issuable upon the exercise of the Pre-Funded Warrants or issuable upon the conversion of the Series B Preferred Stock underlying the Warrants purchased in this offering, other than shares acquired by our current stockholders who are also subject to the contractual lock-up, may be resold into the public market immediately without restriction, unless owned or purchased by our affiliates. Moreover, some of the holders of Common Stock have the right, subject to specified conditions, to require us to file registration statements covering their shares or to include their shares in registration statements that we may file for ourselves or other stockholders.
As of March 31, 2018, there were approximately 1,118,849 shares subject to outstanding options or that are otherwise issuable under our 2013 Equity Compensation Plan, all of which shares we have registered under the Securities Act of 1933, as amended, or the Securities Act, on a registration statement on Form S-8. These shares can be
freely sold in the public market upon issuance, subject to volume limitations applicable to affiliates and the lock-up agreements described above, to the extent applicable.
We do not intend to pay any cash dividends on Common Stock in the foreseeable future and, therefore, any return on your investment in Common Stock must come from increases in the fair market value and trading price of Common Stock.
We do not intend to pay any cash dividends on Common Stock in the ~~Warrants prior~~foreseeable future and, therefore, any return on your investment in Common Stock must come from increases in the fair market value and trading price of Common Stock.
If we issue substantially all of our available authorized shares of Common Stock in this offering, we will not be able to ~~their date~~issue additional shares for future capital raising transactions or strategic transactions unless we obtain stockholders’ approval to amend our certificate of ~~expiration. Any Warrants~~incorporation to increase the number of authorized shares of Common Stock.
We have 100,000,000 authorized shares of Common Stock. As of April 20, 2018, we had 20,243,108 shares of Common Stock outstanding, 1,118,849 shares of Common Stock issuable upon the exercise of outstanding stock options, 15,232,146 shares of Common Stock issuable upon the exercise of outstanding warrants, 33,799 shares of Common Stock reserved for future issuance under our 2013 Equity Compensation Plan and 5,200,000 shares of Common Stock reserved for issuance under our effective registration statement on Form S-1 (Filed No. 333-207533) in connection with our agreement with Lincoln Park. As a result, as of April 20, 2018, we had approximately 58.2 million authorized shares of Common Stock available for issuance. If we issue substantially all of our available authorized shares of Common Stock in this offering which we expect to do, we will not ~~exercised~~be able to issue additional shares for future capital raising transactions or strategic transactions unless we obtain stockholders’ approval to amend our certificate of incorporation to increase the number of authorized shares of Common Stock. This may cause a delay in our future capital raising, collaboration, partnership or other strategic transactions, and may have a material adverse effect on our business and financial condition.
We may issue additional series of preferred stock that rank senior or equally to the Series B Preferred Stock as to dividend payments and liquidation preference.
Neither our certificate of incorporation nor the Certificate of Designation for the Series B Preferred Stock prohibits us from issuing additional series of preferred stock that would rank senior or equally to the Series B Preferred Stock as to dividend payments and liquidation preference. Our certificate of incorporation provides that we have the authority to issue up to 5,000,000 shares of preferred stock, 1,044,488 shares have been designated as Series A Convertible Preferred Stock. The Series B Preferred Stock will rank equally with the Series A Convertible Preferred Stock as to dividend payments and liquidation preference. The issuances of other series of preferred stock could have the effect of reducing the amounts available to the Series B Preferred Stock in the event of our liquidation, winding-up or dissolution. It may also reduce cash dividend payments on the Series B Preferred Stock if we do not have sufficient funds to pay dividends on all Series B Preferred Stock outstanding and outstanding parity preferred stock.
The Series B Preferred Stock will rank junior to all our liabilities to third party creditors in the event of a bankruptcy, liquidation or winding up of our assets.
In the event of bankruptcy, liquidation or winding up, our assets will be available to pay obligations on the Series B Preferred Stock only after all our liabilities have been paid. The Series B Preferred Stock will effectively rank junior to all existing and future liabilities held by ~~their date~~third party creditors. The terms of ~~expiration~~the Series B Preferred Stock do not restrict our ability to raise additional capital in the future through the issuance of debt. In the event of bankruptcy, liquidation or winding up, there may not be sufficient assets remaining, after paying our liabilities, to pay amounts due on any or all of the Series B Preferred Stock then outstanding.
Future issuances of preferred stock may adversely affect the market price for Common Stock.
Additional issuances and sales of preferred stock, or the perception that such issuances and sales could occur, may cause prevailing market prices for Common Stock to decline and may adversely affect our ability to raise additional capital in the financial markets at times and prices favorable to us.
We are not in compliance with the Nasdaq continued listing requirements. If we are unable to comply with the continued listing requirements of the Nasdaq Capital Market, Common Stock could be delisted, which could affect Common Stock’s market price and liquidity and reduce our ability to raise capital.
We are required to meet certain qualitative and financial tests to maintain the listing of our securities on The Nasdaq Capital Market. As previously disclosed, as of March 31, 2017, June 30, 2017, September 30, 2017 and December 31, 2017, our total stockholders’ equity was $(2.7) million, $0.4 million, $(6.1) million and $(10.9) million, respectively. As a result, we did not comply with the Nasdaq’s $2.5 million minimum stockholders’ equity requirement, nor the alternative compliance standards under Nasdaq Listing Rule 5550(b) for the continued listing of our securities on The Nasdaq Capital Market. In addition, as previously disclosed, the Nasdaq Staff notified us of the noncompliance and, after granting a grace period and reviewing our proposed plan to regain compliance, the Nasdaq Staff had determined to seek to delist our securities from Nasdaq unless we requested a hearing before a Nasdaq Hearings Panel (the “Panel”). Accordingly, we requested and had a hearing on January 18, 2018 before the Panel, which has the authority to grant us an additional extension of time to regain compliance.
On February 2, 2018, we received a letter from the Panel stating that the Panel had granted the Company an extension to April 13, 2018 to regain compliance with the continued listing requirements of the Nasdaq Capital Market, which may be accomplished by demonstrating minimum stockholders’ equity of $2.5 million or having a market value of listed securities of at least $35 million for ten consecutive trading days, as defined in Nasdaq Listing Rule 5550(b).
As of April 13, 2018, we were not able to regain compliance. On April 11, 2018, we submitted a written request to the Panel requesting an extension to May 14, 2018 to regain compliance.
On April 23, 2018, we received a letter from the Panel stating that the Panel has granted us an extension to May 14, 2018 to regain compliance.
                There is no assurance that we will ~~expire worthless~~regain compliance on or before May 14, 2018, and even if we do, that we will be ~~under no further obligation~~able to ~~the Warrant holder.~~
~~The Warrants may~~maintain compliance. If we are unable to regain compliance by May 14, 2018 or maintain compliance and our securities are delisted, it could be ~~redeemed on short notice. This may have an adverse impact on their price.~~
        ~~We may redeem~~more difficult to buy or sell our securities and to obtain accurate quotations, and the ~~Warrants for $            per Warrant once the closing~~ price of our ~~common stock has equaled or exceeded $            per share, subject~~securities could suffer a material decline. Delisting could also impair our ability to adjustment, for 10 consecutive trading days. If we give notice of redemption, you will be forced to sell or exercise your Warrants or accept the redemption price. The notice of redemption could come at a time when it is not advisable or possible for you to exercise the Warrants. As a result, you would be unable to benefit from owning the Warrants being redeemed.raise capital.
~~The dealer-manager is not underwriting, nor acting as placement agent of, the Subscription Rights or the securities underlying the Subscription Rights.~~
        Maxim Group LLC is acting as dealer-manager for this Rights Offering. Under the terms and subject to the conditions contained in the dealer-manager agreement, the dealer-manager will provide marketing assistance in connection with this offering. The dealer-manager is not underwriting or placing any of the Subscription Rights or the Units, shares of common stock or Warrants being issued in this offering, and does not make any recommendation with respect to such Subscription Rights (including with respect to the exercise or expiration of such Subscription Rights), Units, shares of common stock or Warrants. The dealer-manager will not be subject to any liability to us in rendering the services contemplated by the dealer-manager agreement except for any act of bad faith or gross negligence by the dealer-manager. The services of the dealer-manager to us in connection with this offering cannot be construed as any assurance that this offering will be successful.
~~Since the Warrants are executory contracts, they may have no value in a bankruptcy or reorganization proceeding.~~
        In the event a bankruptcy or reorganization proceeding is commenced by or against us, a bankruptcy court may hold that any unexercised Warrants are executory contracts that are subject to rejection by us with the approval of the bankruptcy court. As a result, holders of the Warrants may, even if we have sufficient funds, not be entitled to receive any consideration for their Warrants or may receive an amount less than they would be entitled to if they had exercised their Warrants prior to the commencement of any such bankruptcy or reorganization proceeding.

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CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus ~~includes~~and the documents incorporated by reference herein and therein contain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. All statements, other than statements of historical facts, contained in this prospectus and the documents incorporated by reference herein regarding our strategy, future operations, financial position, future revenues, projected costs, prospects, plans and objectives of management are forward-looking statements. We may, in some cases, use terms such as ~~"believes," "estimates," "anticipates," "expects," "plans," "intends," "may," "could," "might," "should," "approximately"~~“believes,” “estimates,” “anticipates,” “expects,” “plans,” “intends,” “may,” “could,” “might,” “will,” “should,” “approximately” or other words that convey uncertainty of future events or outcomes to identify these forward-looking statements. Forward-looking statements appear in a number of places throughout this ~~report~~prospectus and the documents incorporated by reference herein, and include statements regarding our intentions, beliefs, projections, outlook, analyses or current expectations concerning, among other things, our ongoing and planned preclinical development and clinical trials, the timing of and our ability to make regulatory filings and obtain and maintain regulatory approvals for our product candidates, protection of our intellectual property portfolio, the degree of clinical utility of our products, particularly in specific patient populations, our ability to develop commercial and manufacturing functions, expectations regarding clinical trial data, our results of operations, cash needs, financial condition, liquidity, prospects, growth and strategies, the industry in which we operate and the trends that may affect the industry or us.

By their nature, forward-looking statements involve risks and uncertainties because they relate to events, competitive dynamics and industry change, and depend on the economic circumstances that may or may not occur in the future or may occur on longer or shorter timelines than anticipated. Although we believe that we have a reasonable basis for each forward-looking statement contained in this ~~report,~~prospectus and in documents incorporated by reference herein, we caution you that forward-looking statements are not guarantees of future performance and that our actual results of operations, financial condition and liquidity, and the development of the industry in which we operate may differ materially from the forward-looking statements contained in this report. In addition, even if our results of operations, financial condition and liquidity, and events in the industry in which we operate are consistent with the forward-looking statements contained in this report, they may not be predictive of results or developments in future periods.prospectus.

Actual results could differ materially and adversely from our forward-looking statements due to a number of factors, including, without limitation, risks related to:
•
·our need for additional financing for our INSPIRE trial and other operations, and our ability to obtain sufficient funds on acceptable terms when needed, and our plans and future needs to scale back operations if adequate financing is not obtained;
·

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our ability to continue as a going concern;
·

•
our estimates regarding expenses, future revenues, capital requirements and needs for additional financing;
·

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the success and timing of our preclinical studies and clinical trials, including site initiation and patient enrollment, and regulatory approval of protocols for future clinical trials;
·

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our ability to enter into, maintain and perform collaboration agreements with other pharmaceutical companies, for funding and commercialization of our clinical drug product candidates or preclinical compounds, and our ability to achieve certain milestones under those agreements;
·

•
the difficulties in obtaining and maintaining regulatory approval of our product candidates, and the labeling under any approval we may obtain;
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our plans and ability to develop, manufacture and commercialize our product candidates;
·

•
our failure to recruit or retain key scientific or management personnel or to retain our executive officers;
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the size and growth of the potential markets for our product candidates and our ability to serve those markets;
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•
regulatory developments in the United States and foreign countries;

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•
the rate and degree of market acceptance of any of our product candidates;
·

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obtaining and maintaining intellectual property protection for our product candidates and our proprietary technology;
·

•
the successful development of our commercialization capabilities, including sales and marketing capabilities;
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recently enacted and future legislation and regulation regarding the healthcare system;
·

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the success of competing therapies and products that are or may become available;
·

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our ability to maintain the listing of our ~~common stock~~securities on a national securities exchange;
·

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the potential for third party disputes and litigation; and
·

•
the performance of third parties, including contract research organizations ~~or CROs~~ and third-party manufacturers.

Any forward-looking statements that we make in this prospectus and the documents incorporated by reference herein speak only as of the date of such statement, and we undertake no obligation to update such statements ~~to reflect~~whether as a result of any new information, future events, changed circumstances or ~~circumstances after the date of this prospectus or to reflect the occurrence of unanticipated events.~~otherwise. Comparisons of results for current and any prior periods are not intended to express any future trends or indications of future performance, unless expressed as such, and should only be viewed as historical data.

You should also read carefully the factors described in the ~~"Risk Factors"~~“Risk Factors” section of this prospectus and in ~~our annual report on Form 10-K filed with the SEC on March 28, 2016,~~documents incorporated by reference herein, to better understand ~~significant~~the risks and uncertainties inherent in our business and underlying any forward-looking statements. As a result of these factors, ~~actual results could differ materially and adversely from those anticipated or implied in~~we cannot assure you that the forward-looking statements in this ~~report~~prospectus and in documents incorporated by reference herein will prove to be accurate. Furthermore, if our forward-looking statements prove to be inaccurate, the inaccuracy may be material. In light of the significant uncertainties in these forward-looking statements, you should not ~~place undue reliance on~~regard these statements as a representation or warranty by us or any ~~forward-looking statements.~~other person that we will achieve our objectives and plans in any specified timeframe, or at all.
We obtained the industry, market and competitive position data in this prospectus and in documents incorporated by reference herein from our own internal estimates and research as well as from industry and general publications and research surveys and studies conducted by third parties. Industry publications and surveys generally state that the information contained therein has been obtained from sources believed to be reliable. We believe this data is accurate in all material respects as of the date of this prospectus.

USE OF PROCEEDS
~~Assuming~~
We estimate that ~~all~~the net proceeds to us from this offering will be approximately $13.4 million, based on an assumed public offering price per Unit of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018, assuming the sale of 26,785,714 Units ~~are sold~~and no sale of any Pre-Funded Units in this offering, after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us, and excluding the ~~Rights Offering,~~proceeds, if any, from the exercise of Warrants issued pursuant to this offering. If the underwriter exercises its option to purchase the additional Units in full, we estimate that the net proceeds ~~from the Rights Offering~~ will be approximately $$15.4 million, based on ~~the Subscription Price of $~~an assumed public offering price per Unit of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018, assuming the sale of 30,803,571 Units and no sale of any Pre-Funded Units in this offering, after deducting ~~fees~~estimated underwriting discounts and ~~expenses payable to the dealer-manager,~~commissions and ~~after deducting other~~estimated offering expenses payable by us, and excluding the proceeds, if any, ~~proceeds received upon~~from the exercise of ~~any~~the Warrants issued ~~in the Rights Offering.~~pursuant to this offering.

We intend to use the net proceeds from this ~~Rights Offering primarily~~offering to continue funding our ongoing Phase 3 clinical trial of rigosertib IV in a population of patients with higher-risk MDS after failure of HMA therapy, which we refer to as "INSPIRE," and, to a lesser extent, for otherfund the development of our clinical and preclinical programs, for other research and development activities ~~business development~~ and for general corporate purposes, which may include capital expenditures and funding our working capital needs.
        ~~Our management will have broad discretion in~~ We expect from time to time to evaluate the ~~application~~acquisition of businesses, products and technologies for which a portion of the net proceeds may be used, although we currently are not planning or negotiating any such transactions.
The amounts actually expended for each purpose may vary significantly depending upon numerous factors, including the amount and timing of the proceeds from this offering and ~~investors~~progress with the clinical development of our product candidates. Expenditures will also depend upon the establishment of collaborative arrangements with other companies, the availability of additional financing and other factors. Investors will be relying on the judgment of our management regarding the application of the proceeds of any sale of shares of our securities.
As of the date of this prospectus, we cannot specify with ~~regard to~~certainty all of the particular uses of the proceeds from this offering. Accordingly, we will retain broad discretion over the use of ~~these net~~such proceeds. Pending the use of the net proceeds from this offering as described above, we intend to ~~hold~~invest the net proceeds in ~~cash or invest in short-term,~~ investment-grade, interest-bearing ~~instruments.~~securities.

The actual offering price per Unit and Pre-Funded Unit, and the exercise price of the Warrants, as applicable, will be as determined by negotiation between us and the underwriter at the time of pricing, and may be at a discount to the current market price of Common Stock. These estimates exclude the proceeds, if any, from the exercise of the Warrants in this offering. If all of the Warrants sold in this offering were to be exercised in cash at an assumed exercise price of $0.56 per 0.025 share of Series B Preferred Stock, we would receive additional net proceeds of approximately $15.0 million. However, the Warrants contain a cashless exercise provision that permits exercise of the Warrants on a cashless basis (i) at any time when there is no effective registration statement under the Securities Act of 1933, as amended, covering the issuance of the underlying shares of Series B Preferred Stock or (ii) on the expiration date of the Warrant. We cannot predict when or if the Warrants will be exercised or whether they will be exercised for cash. It is possible that the Warrants may be exercised solely on a cashless basis.
A $0.25 increase or decrease in the assumed public offering price per Unit of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018, would increase or decrease the net proceeds to us from this offering by $6.2 million, assuming that the number of Units offered by us, as set forth on the cover page of this prospectus, remains the same, assuming no sale of any Pre-Funded Units, after deducting the estimated underwriter discounts and commissions and estimated offering expenses payable by us, and excluding the proceeds, if any, from the exercise of the Warrants issued pursuant to this offering.
Similarly, each increase or decrease of 1,000,000 Units offered by us would increase or decrease the net proceeds to us by approximately $0.5 million, assuming the assumed public offering price per Unit of $0.56 remains the same, assuming no sale of any Pre-Funded Units, after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, and excluding the proceeds, if any, from the exercise of the Warrants issued pursuant to this offering.
~~Table of Contents~~CAPITALIZATION

~~CAPITALIZATION~~

The following table presents our cash, cash equivalents and capitalization, as of ~~March~~December 31, ~~2016:~~2017:
•
·on an actual basis;
·                  on a pro forma basis to give effect to (i) our underwritten public offering (the “February 2018 Offering”) of 7,005,000 shares of Common Stock, pre-funded warrants to purchase 2,942,500 shares of Common Stock and preferred stock warrants to purchase 994,750 shares of our Series A Convertible Preferred Stock pursuant to an underwriting agreement between us and H.C. Wainwright & Co., LLC dated as of February 8, 2018 and (ii) subsequent exercises of pre-funded warrants to purchase 1,650,000 shares of Common Stock (together with the February 2018 Offering, the “Prior Transactions”); and
·

•
on a pro forma as adjusted basis to give further effect to the sale ~~by us~~of 26,785,714 Units in this ~~Rights Offering of maximum of                Units (consisting of                shares of our common stock and Warrants to purchase~~offering at an ~~aggregate of                shares of common stock upon exercise), at the Subscription Price of $~~assumed public offering price per Unit ~~and our receipt~~ of $0.56, the ~~net proceeds from that~~last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018, assuming no sale of any Pre-Funded Warrants, after deducting estimated underwriting discounts and commissions and estimate offering ~~expenses.~~
        ~~The table below does not reflect~~expenses payable by us, and excluding the proceeds, if any, from the exercise of the Warrants issued in connection with the Rights Offering. The pro forma as adjusted information set forth below assumes equity accounting for the Warrants, is illustrative only and will be adjusted based on the Units sold. pursuant to this offering.
You should read this information in conjunction with our consolidated financial statements and notes thereto incorporated by reference into this prospectus.

December 31, 2017 (unaudited)

Pro Forma

Actual

Pro Forma

as Adjusted

Cash and cash equivalents

$
4,024,000

$
12,724,000

$
26,075,000

Long-term liabilities

5,864,000

5,864,000

5,864,000

Stockholders’ (deficit) equity:

Preferred stock, $0.01 par value, 5,000,000 authorized, none issued and outstanding, actual, pro forma and pro form as adjusted

Common stock, $0.01 par value, 25,000,000 authorized, 10,771,163 issued and outstanding, actual; 25,000,000 authorized, 19,426,163 shares issued and outstanding, pro forma; 100,000,000 shares authorized, 46,211,877 issued and outstanding, pro forma as adjusted (1)

108,000

194,000

462,000

Additional paid-in capital

350,514,000

359,128,000

372,211,000

Accumulated other comprehensive loss

3,000

3,000

3,000

Accumulated deficit

(362,316,000
)
(362,316,000
)
(362,316,000
)
Total Onconova Therapeutics, Inc. stockholders’ (deficit) equity

(11,691,000
)
(2,991,000
)
10,360,000

Non-controlling interest

830,000

830,000

830,000

Total stockholders’ (deficit) equity

(10,861,000
)
(2,161,000
)
11,190,000

March 31, 2016 (unaudited)

Actual Pro Forma
as Adjusted
Cash and cash equivalents
$ 16,835,000 $ —
Long-term liabilities
5,181,000
Stockholders' equity*:

Preferred stock, $0.01 par value, 5,000,000 authorized at March 31, 2016 and March 31, 2016 Pro Forma, none issued and outstanding at March 31, 2016
— —
Common stock, $0.01 par value, 25,000,000 authorized at March 31, 2016 and March 31, 2016 Pro Forma as Adjusted, 2,740,103 shares issued and outstanding at March 31, 2016 and                shares issued and outstanding at March 31, 2016 Pro Forma
27,000 —
Additional paid-in capital
331,775,000 —
Accumulated other comprehensive loss
(16,000 ) —
Accumulated deficit
(325,797,000 ) —

Total Onconova Therapeutics, Inc. stockholders' equity
5,989,000 —
Non-controlling interest
830,000 —

Total stockholders' equity
6,819,000 —

*
~~Stockholders'sequity numbers have been restated~~
(1) On March 21, 2018, we filed with the Secretary of State of the State of Delaware a Certificate of Amendment to ~~reflect~~our Tenth Amended and Restated Certificate of Incorporation, as amended to increase the ~~one-for-ten reverse~~number of authorized shares of our capital stock ~~split and decrease~~from 30,000,000 shares to 105,000,000 shares in order to increase the number of authorized ~~common~~ shares ~~from 75,000,000~~of Common Stock 25,000,000 shares to ~~25,000,000.~~
100,000,000 shares.

The ~~information~~ above table is based on 10,771,163 shares of Common Stock outstanding as of ~~March~~December 31, ~~2016~~2017 and excludes:
•
~~567,122~~·                  894,996 shares of ~~common stock~~Common Stock issuable upon the exercise of stock options outstanding at ~~March~~December 31, ~~2016~~2017 with a weighted average exercise price of ~~$72.60~~approximately $40.41 per share;
·

•
~~97,424~~                  3,294,771 shares of ~~common stock~~Common Stock issuable upon the exercise of outstanding warrants at ~~March~~December 31, ~~2016~~2017 with a weighted average exercise price of ~~$12.05~~approximately $5.10 per ~~share;~~

•
~~185,909~~share and, on a pro forma basis giving effect to the Prior Transactions, 15,032,146 shares of ~~common~~Common Stock issuable upon the exercise of outstanding warrants at December 31, 2017 with a weighted average exercise price of approximately $1.83 per share (includes Common Stock issuable for warrants which are exercisable for our Series A Convertible Preferred Stock, which is convertible to Common Stock); to the extent the preferred stock warrant repricing described under “Underwriting — Company Lock-up Waiver Agreement” occurs, preferred stock warrants to purchase an aggregate of 994,750 shares of our Series A Convertible Preferred Stock, which are convertible into an aggregate of 9,947,500 shares of Common Stock, will be repriced, and the weighted average exercise price of all outstanding warrants at December 31, 2017, on a pro forma basis giving effect to the Prior Transactions, taking into account such repricing will be approximately $1.55 per share based on an assumed public offering price per Unit of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018;
·                  57,632 shares of Common Stock reserved for future issuance under our 2013 Equity Compensation Plan at ~~March~~December 31, ~~2016; and~~2017;
·

•
any additional shares of ~~common stock~~Common Stock that we may issue to ~~Lincoln Park Capital Fund, LLC, or~~ Lincoln Park, pursuant to a purchase agreement we entered into on October 8, 2015, which

~~Table of Contents~~
provides that, upon the terms and subject to the conditions and limitation set forth therein, Lincoln Park is committed to purchase up to an aggregate of an additional $15 million of shares of ~~our common stock~~Common Stock over the term of the purchase agreement, should we elect to sell shares to Lincoln ~~Park.~~Park; and
~~All share and per share data has been restated to reflect our one-for-ten reverse stock split effective May 31, 2016, subject to final adjustments for treatment of fractional share interests.~~

·         ~~DILUTION~~
shares of Common Stock issuable upon the exercise of the Underwriter’s Warrants issued in this offering.
~~Purchasers~~
Each $0.25 increase or decrease in the assumed public offering price per Unit of $0.56, the last reported sale price of our ~~common stock in~~Common Stock on the ~~Rights Offering (and upon~~Nasdaq Capital Market on April 20, 2018, would increase or decrease the net proceeds to us from this offering by $6.2 million, assuming that the number of Units offered by us, as set forth on the cover page of this prospectus, remains the same, assuming no sale of any Pre-Funded Units, after deducting the estimated underwriter discounts and commissions and estimated offering expenses payable by us, and excluding the proceeds, if any, from the exercise of the Warrants issued pursuant to this ~~Rights Offering)~~offering. We may also increase or decrease the number of Units offered in this offering. Each increase or decrease of 1,000,000 Units offered by us would increase or decrease the net proceeds to us by approximately $0.5 million, assuming the assumed public offering price per Unit of $0.56 remains the same, assuming no sale of any Pre-Funded Units, after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, and excluding the proceeds, if any, from the exercise of the Warrants issued pursuant to this offering. The as adjusted information discussed above is illustrative only and will ~~experience an immediate dilution~~be adjusted based on the actual public offering price and other terms of this offering as determined between us and the underwriter at pricing.
DILUTION
If you invest in Common Stock in this offering, your interest will be diluted to the extent of the difference between the effective public offering price per share of Common Stock included in the Units or issuable upon the exercise of the Pre-Funded Warrants and the pro forma as adjusted net tangible book value per share of Common Stock after this offering. As of December 31, 2017, our ~~common stock.~~historical net tangible book value was $(10.9) million, or $(1.01) per share, based on 10,771,163 shares of Common Stock outstanding as of December 31, 2017. Our historical net tangible book value per share represents the amount of our total tangible assets reduced by the amount of our total liabilities, divided by the total number of shares of Common Stock outstanding as of December 31, 2017.
On a pro forma basis, after giving effect to the Prior Transactions, our net tangible book value at December 31, 2017 would have been $(2.2) million, or $(0.11) per share.
After giving further effect to our sale in this offering of 26,785,714 Units at an assumed public offering price per Unit of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018, assuming no sale of any Pre-Funded Units in this offering, and after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, and excluding the proceeds, if any, from the exercise of the Warrants issued in this offering, our pro forma as adjusted net tangible book value as of ~~March~~December 31, ~~2016 was approximately $6,819,000,~~2017 would have been $11.2 million, or ~~$2.49~~$0.24 per ~~share~~share. This represents an immediate increase of our common stock (based upon 2,740,104 shares of our common stock outstanding after giving effect to the one-for-ten reverse stock split on May 31, 2016). Net tangible book value per share is equal to our totalpro forma net tangible book value ~~which is~~of $0.35 per share to our ~~total~~existing stockholders and an immediate dilution of pro forma net tangible ~~assets less our total liabilities, divided by~~book value of $0.32 per share to investors purchasing Units in this offering. The following table illustrates this per share dilution.
Assumed public offering price per Unit

$
0.56

Historical net tangible book value (deficit) per share as of December 31, 2017

$
(1.01
)

Pro forma increase in net tangible book value per share attributable to the Prior Transactions

0.90

Pro forma net tangible book value per share December 31, 2017

(0.11
)

Increase in pro forma net tangible book value per share attributable to investors purchasing in this offering

0.35

Pro forma as adjusted net tangible book value per share as of December 31, 2017 after this offering

0.24

Dilution per share to investors purchasing in this offering

$
0.32

Each $0.25 increase or decrease in the ~~number~~assumed public offering price per Unit of ~~shares~~$0.56, the last reported sale price of our ~~outstanding common stock. Dilution per share equals~~Common Stock on the ~~difference between~~Nasdaq Capital Market on April 20, 2018, would increase or decrease the ~~amount per share paid~~net proceeds to us from this offering by ~~purchasers of shares of common stock in the Rights Offering and the~~$6.2 million, increase or decrease our pro forma as adjusted net tangible book value per share after this offering by approximately $0.13, and increase or decrease the dilution per share to new investors purchasing in this offering by $0.12, assuming that the number of ~~our common stock immediately after the Rights Offering.~~
        ~~Based~~Units offered by us, as set forth on the cover page of this prospectus, remains the same, assuming no sale ~~by us in this Rights Offering~~ of ~~a maximum of~~any Pre-Funded Units, (consisting of                shares of our common stock and Warrants to purchase an aggregate of                 shares of common stock upon exercise), at the Subscription Price of $            per Unit, and after deducting the estimated underwriter discounts and commissions and estimated offering ~~expenses and dealer-manager fees and~~ expenses payable by us, ~~of $                ,~~ and excluding the ~~application~~proceeds, if any, from the exercise of the ~~estimated $~~Warrants issued pursuant to this offering. We may also increase or decrease the number of Units offered in this offering. Each increase or decrease of 1,000,000 Units offered by us would increase or decrease the net proceeds ~~from the Rights Offering, assuming equity accounting for the Warrants,~~to us by approximately $0.5 million, increase or decrease our pro forma as adjusted net tangible book value per share after this offering by $0.01, and increase or decrease the dilution per share to new investors purchasing in this offering by $0.01, assuming the assumed public offering price per Unit of $0.56 remains the same, assuming no sale of any Pre-Funded Units, after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, and excluding the proceeds, if any, from the exercise of the Warrants issued pursuant to this offering. The as adjusted information discussed above is illustrative only and will be adjusted based on the actual public offering price and other terms of ~~March 31, 2016~~this offering as determined between us and the underwriter at pricing.
If the underwriter exercises its option to purchase additional Units in full, and assuming no sale of any Pre-Funded Units in this offering, the pro forma as adjusted net tangible book value per share after this offering would ~~have been approximately $                , or $~~be $0.26 per ~~share. This represents an immediate~~share, the increase in pro forma net tangible book value per share to existing ~~shareholders of $~~stockholders would be $0.37 per share and ~~an immediate~~the dilution to ~~purchasers~~new investors purchasing Units in ~~the Rights Offering of $~~this offering would be $0.30 per share.

The ~~following~~above discussion and table ~~illustrates this per-share dilution~~are based on a10,771,163 shares of Common Stock outstanding as of December 31, 2017, actual, and 19,426,163 shares of Common Stock outstanding as of December 31, 2017 pro forma ~~basis, assuming a fully subscribed for Rights Offering of                Units at the Subscription Price of $            per Unit (but excluding any issuance of~~and exclude:
·                  894,996 shares of ~~common stock upon exercise of Warrants):~~

Subscription Price
$
Net tangible book value per share as of March 31, 2016, before Rights Offering
$ 2.49
Increase in net tangible book value per share attributable to Rights Offering

Pro forma net tangible book value per share as of March 31, 2016, after giving effect to Rights Offering

Dilution in net tangible book value per share to purchasers
$
        ~~The information above is as of March 31, 2016 and excludes:~~
•
~~567,122 shares of common stock~~Common Stock issuable upon the exercise of stock options outstanding at ~~March~~December 31, ~~2016~~2017 with a weighted average exercise price of ~~$72.60~~approximately $40.41 per share;
·

•
~~97,424~~                  3,294,771 shares of ~~common stock~~Common Stock issuable upon the exercise of outstanding warrants at ~~March~~December 31, ~~2016~~2017 with a weighted average exercise price of ~~$12.05~~approximately $5.10 per ~~share;~~

•
~~185,909~~share and, on a pro forma basis giving effect to the Prior Transactions, 15,032,146 shares of ~~common~~Common Stock issuable upon the exercise of outstanding warrants at December 31, 2017 with a weighted average exercise price of approximately $1.83 per share (includes Common Stock issuable for warrants which are exercisable for our Series A Convertible Preferred Stock, which is convertible to Common Stock); to the extent the preferred stock warrant repricing described under “Underwriting — Company Lock-up Waiver Agreement” occurs, preferred stock warrants to purchase an aggregate of 994,750 shares of our Series A Convertible Preferred Stock, which are convertible into an aggregate of 9,947,500 shares of Common Stock, will be repriced, and the weighted average exercise price of all outstanding warrants at December 31, 2017, on a pro forma basis giving effect to the Prior Transactions, taking into account such repricing will be approximately $1.55 per share based on an assumed public offering price per Unit of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018;
·                  57,632 shares of Common Stock reserved for future issuance under our 2013 Equity Compensation Plan at ~~March~~December 31, ~~2016; and~~2017;
·

•
any additional shares of ~~common stock~~Common Stock that we may issue to ~~Lincoln Park Capital Fund, LLC, or~~ Lincoln Park, pursuant to a purchase agreement we entered into on October 8, 2015, which

~~Table of Contents~~
provides that, upon the terms and subject to the conditions and limitation set forth therein, Lincoln Park is committed to purchase up to an aggregate of an additional $15 million of shares of ~~our common stock~~Common Stock over the term of the purchase agreement, should we elect to sell shares to Lincoln ~~Park.~~
        ~~All share~~Park; and ~~per share data has been restated to reflect our one-for-ten reverse stock split effective May 31, 2016, subject to final adjustments for treatment of fractional share interests.~~

·         shares of Common Stock issuable upon the exercise of the Underwriter’s Warrants issued in this offering.
To the extent that outstanding options or warrants are exercised, ~~the investor purchasing our common stock in this offering~~you will experience further dilution. In addition, we may choose to raise additional capital due to market conditions or strategic ~~considerations.~~considerations even if we believe we have sufficient funds for our current or future operating plans. To the extent that additional capital is raised through the sale of equity or convertible debt securities, the issuance of ~~those~~these securities could result in further dilution to our stockholders.
EXECUTIVE COMPENSATION
~~MARKET PRICE OF OUR COMMON STOCK AND RELATED STOCKHOLDER MATTERS~~
Our common stock began trading on the NASDAQ Global Select Market on July 25, 2013 under the symbol "ONTX." Prior to that time, there was no public market for our common stock. On February 5, 2016, we transferred the listing
Overview of Executive Compensation
The compensation committee of our ~~shares~~board of directors is responsible for overseeing the compensation of all of our executive officers. In this capacity, our compensation committee annually reviews and approves the compensation of our chief executive officer and other executive officers, including such goals and objectives relevant to the ~~NASDAQ Capital Market.~~ executive officers’ compensation that the committee, in its discretion, determines are appropriate, evaluates their performance in light of those goals and objectives, and sets their compensation based on this evaluation.
2017 Summary Compensation Table
The following table sets forth information for the ~~periods indicated the high~~fiscal years ended December 31, 2017 and ~~low intra-day sale prices per share~~2016 concerning compensation of our ~~common~~principal executive officer and the two most highly compensated executive officers during 2017. We refer to these three executive officers as our “named executive officers.”

Option

All Other

Salary

Bonus

Awards

Compensation

Total

Name and Principal Position

Year

($)

($)(1)

($)(2)

($)(3)

($)

Ramesh Kumar, Ph.D.

2017

538,150

—

81,890

23,581

643,621

President and Chief Executive Officer

2016

413,172

254,028

177,729

19,707

864,636

Steven M. Fruchtman, M.D.

2017

436,154

—

49,105

19,315

504,574

Chief Medical Officer and Senior Vice President, Research and Development

2016

421,784

142,800

119,283

7,179

691,046

Manoj Maniar, Ph.D.

2017

388,977

—

36,108

14,410

439,495

Senior Vice President, Product Development

2016

371,453

126,186

89,703

11,930

599,272

(1) Represents discretionary annual bonus amounts paid.
(2) The entries in the option awards column reflect the grant date fair value of the awards, as calculated for financial statement reporting purposes in    accordance with Accounting Standards Codification (ASC) No. 718, Compensation—Stock Compensation. The option values were calculated using the Black-Scholes option pricing model. These amounts do not represent the actual value realized by the named executive officers. See Note 10 of the Notes to Consolidated Financial Statements for the fiscal year ended December 31, 2016 for a discussion of the relevant assumptions used to determine the valuation of our stock ~~as reported~~options for accounting purposes.
(3) Includes amounts paid for insurance premiums on behalf of the ~~NASDAQ Global Market or NASDAQ Capital Market, as adjusted to give effect~~named executive officer and matching funds paid pursuant to our ~~one-for-ten reverse~~401(k) Plan.
Employment Agreements
We have entered into employment agreements with each of our named executive officers, and the compensation of our named executive officers is determined, in large part, by the terms of those employment agreements. Following are descriptions of the material terms of each named executive officer’s employment agreement.
Ramesh Kumar, Ph.D.
We entered into an employment agreement with Dr. Kumar on July 1, 2015, which supersedes any prior employment agreements. The employment agreement continues indefinitely, unless terminated in accordance with the terms of the agreement.
The employment agreement provided for an initial base salary of $543,375, subject to adjustment upon annual review by our board of directors, and an annual bonus of up to 55% of such base salary, payable upon our achievement of revenue or profit objectives, specific business plan goals or other performance milestones mutually agreed to by Dr. Kumar and our board of directors, provided that Dr. Kumar remain employed by us throughout the performance year. The bonus may be paid in the form of cash, stock ~~split effective May 31, 2016:~~

High Low
Year Ending December 31, 2016

First Quarter
$ 10.28 $ 3.23
Second Quarter (through June 1, 2016)
6.40 3.80
Year Ended December 31, 2015

First Quarter
$ 44.30 $ 21.50
Second Quarter
30.20 22.60
Third Quarter
40.00 13.20
Fourth Quarter
18.90 9.20
Year Ended December 31, 2014

First Quarter
$ 162.20 60.50
Second Quarter
64.90 41.00
Third Quarter
57.80 42.40
Fourth Quarter
50.00 32.40
        Inoptions, shares of Common Stock, or a combination thereof, at our compensation committee’s discretion. Dr. Kumar may also be entitled to additional compensation in recognition of extraordinary contributions, at the sole discretion of our compensation committee. On February 12, 2016, we ~~transferred~~entered into a letter agreement with Dr. Kumar pursuant to which Dr. Kumar agreed to a voluntary reduction in his base salary from $543,375 to $407,531, effective as of January 1, 2016. For purposes of severance and other benefits calculated based upon base salary, however, Dr. Kumar’s base salary was deemed to remain at $543,375. On December 9, 2016, our board of directors approved the ~~listing~~termination of ~~our common stock from~~ the ~~NASDAQ Global Select Market~~voluntary salary reduction effective January 1, 2017. Pursuant to this approval, on March 27, 2017, we entered into a letter agreement with Dr. Kumar under which the NASDAQ Capital Market and subsequently received a deficiency letter from NASDAQ notifying us that we had failed to meet the minimum bid price required for continued listing for 30 consecutive business days. In accordance with NASDAQ listing rules, we have been provided an initial period of 180 calendar days, or until August 8, 2016, to regain compliance. On Junevoluntary salary reduction was terminated effective January 1, 2016, the last reported sale price of our common stock on the NASDAQ Capital Market was $4.54 per share. As of May 31 , 2016, we had 167 holders of record of our common stock. The actual number of holders of common stock is greater than these numbers of record holders and includes stockholders who are beneficial owners, but whose shares are held in street name by brokers and nominees. The number of holders of record also does not include stockholders whose shares may be held in trust by other entities.

~~Table of Contents~~

~~DIVIDEND POLICY~~
2017.
We have never declared or paid any cash dividends on our capital stock. We currently intend to retain all available funds and any future earnings to support our corporation and finance the growth and development of our business. We do not intend to pay cash dividends on our common stock for the foreseeable future.

~~THE RIGHTS OFFERING~~
~~The Subscription Rights~~
        ~~We are distributing to the record holders and to holders of certain outstanding warrants who are~~Dr. Kumar is entitled to participate in ~~this offering~~all of our employee benefit plans and programs that are made generally available from time to time to our executive officers and is entitled to vacation benefits. Pursuant to his employment agreement, Dr. Kumar is entitled to term life insurance coverage in a face amount that is not less than his base salary, a reasonable transportation allowance if we relocate our research facility more than 40 miles from its present location, and up to $10,000 annually for educational programs related to the performance of his duties. If Dr. Kumar dies during his employment, we will be entitled to a $1 million death benefit under a “key man” life insurance policy. Dr. Kumar’s employment agreement contains non-solicitation, non-competition, confidentiality and inventions assignment provisions that, among other things, prevent him from competing with us during the term of his employment and for a specified time thereafter.
If Dr. Kumar’s employment is terminated due to his death, disability, by us for “cause” or by Dr. Kumar without “good reason” during the term of his employment agreement, we will pay to Dr. Kumar or his spouse or estate the balance of his accrued and unpaid salary, unreimbursed expenses, and unused accrued vacation time through the termination date.
If Dr. Kumar’s employment is terminated by us without “cause” or by Dr. Kumar for “good reason,” other than during a change in control protection period, Dr. Kumar will be entitled to receive severance equal to his current base salary and target bonus for the fiscal year during which his employment ceases. If the termination is during a change in control protection period, Dr. Kumar will be entitled to receive severance equal to two times the sum of his current base salary and target bonus for the fiscal year during which his employment ceases, less any severance previously paid. A change in control protection period commences three months prior to and ends twelve months following a change in control. The Company will also reimburse Dr. Kumar for a portion of his medical insurance costs and all of Dr. Kumar’s incentive stock options that are unvested as of the date of such termination would fully vest as of the date of termination.
Steven Fruchtman, M.D.
We entered into an employment agreement with Dr. Fruchtman on July 1, 2015, which supersedes any prior employment agreements. The employment agreement continues indefinitely, unless terminated in accordance with the terms of the agreement.
The employment agreement provides for an initial base salary of $420,000, subject to adjustment upon annual review, and subject to the compensation committee’s sole discretion, an annual bonus, based on the performance of Dr. Fruchtman and the Company, of up to 40% of such base salary. The bonus may be paid in the form of cash, stock options, shares of Common Stock, or a combination thereof, at our compensation committee’s discretion.
Dr. Fruchtman is entitled to participate in all of our employee benefit plans and programs that are made generally available from time to time to our executive officers and is entitled to vacation benefits. Dr. Fruchtman’s employment
agreement contains non-solicitation, non-competition, confidentiality and inventions assignment provisions that, among other things, prevent him from competing with us during the term of his employment and for a specified time thereafter. The Company will reimburse Dr. Fruchtman for reasonable expenses including certain commuting costs to the Company’s offices.
If Dr. Fruchtman’s employment is terminated due to his death, disability, by us for “cause” or by Dr. Fruchtman without “good reason” during the term of his employment agreement, we will pay to Dr. Fruchtman or his spouse or estate the balance of his accrued and unpaid salary, unreimbursed expenses, and unused accrued vacation time through the termination date.
If Dr. Fruchtman’s employment is terminated by us without “cause” or by Dr. Fruchtman for “good reason,” other than during a change in control protection period, Dr. Fruchtman will be entitled to receive severance equal to the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. If the termination is during a change in control protection period, Dr. Fruchtman will be entitled to receive severance equal to the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. A change in control protection period is the twelve months following a change in control. The Company will also reimburse Dr. Fruchtman for a portion of his medical insurance costs and all of Dr. Fruchtman’s incentive stock options that are unvested as of the date of such termination would fully vest as of the date of termination.
Manoj Maniar, Ph.D.
We entered into an employment agreement with Dr. Maniar on July 1, 2015, which supersedes any prior employment agreements. The employment agreement continues indefinitely, unless terminated in accordance with the terms of the agreement.
The employment agreement provides for an initial base salary of $371,135, subject to adjustment upon annual review by our board of directors, and subject to the compensation committee’s sole discretion, an annual bonus, based on the performance of Dr. Maniar and the Company, of up to 40% of such base salary. The bonus may be paid in the form of cash, stock options, shares of Common Stock, or a combination thereof, at our compensation committee’s discretion.
Dr. Maniar is entitled to participate in all of our employee benefit plans and programs that are made generally available from time to time to our executive officers and is entitled to vacation benefits. Dr. Maniar’s employment agreement contains non-solicitation, non-competition, confidentiality and inventions assignment provisions that, among other things, prevent him from competing with us during the term of his employment and for a specified time thereafter.
If Dr. Maniar’s employment is terminated due to his death, disability, by us for “cause” or by Dr. Maniar without “good reason” during the term of his employment agreement, we will pay to Dr. Maniar or his spouse or estate the balance of his accrued and unpaid salary, unreimbursed expenses, and unused accrued vacation time through the termination date.
If Dr. Maniar’s employment is terminated by us without “cause” or by Dr. Maniar for “good reason,” other than during a change in control protection period, Dr. Maniar will be entitled to receive severance equal to nine-twelfths of the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. If the termination is during a change in control protection period, Dr. Maniar will be entitled to receive severance equal to the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. A change in control protection period is the twelve months following a change in control. The Company will also reimburse Dr. Maniar for a portion of his medical insurance costs and all of Dr. Maniar’s incentive stock options that are unvested as of the date of such termination would fully vest as of the date of termination.
Stock Option and Other Compensation Plans
We maintain our 2013 Equity Compensation Plan for the purpose of attracting key employees, directors and consultants, inducing them to remain with us and encouraging them to increase their efforts to make our business more
successful. The plan provides for awards of stock options, stock appreciation rights, restricted stock, restricted stock Units, deferred shares and other equity-based awards.
The following table contains certain information regarding equity awards held by the named executive officers as of December 31, 2017:
Outstanding Equity Awards at 2017 Fiscal Year-End
Name

Number of
Securities
Underlying
Unexercised
Options (#)
Exercisable

Number of
Securities
Underlying
Unexercised
Options (#)
Unexercisable

Option
Exercise
Price
($)

Option
Expiration
Date

Ramesh Kumar

9,376

—

57.60

3/16/2020

5,251

—

61.30

12/9/2020

1,033

—

61.30

12/4/2021

18,754

—

132.80

12/18/2022

10,500

—

150.00

7/25/2023

13,500
(1)
—

134.80

12/20/2023

13,125
(1)
4,375

39.80

12/17/2024

5,833
(1)
2,917

23.20

4/15/2025

4,921
(1)
3,829

14.80

9/24/2025

14,644

—

6.50

1/25/2026

18,333
(2)
25,667

3.24

9/1/2026

4,224
(2)
8,449

2.65

12/15/2026

13,452
(2)
30,575

2.70

1/17/2027

Steven Fruchtman

8,750
(1)
3,250

43.70

1/11/2025

2,333
(1)
1,167

24.80

4/19/2025

2,250
(1)
1,750

14.80

9/24/2025

12,410

—

6.50

1/25/2026

10,416
(2)
14,584

3.24

9/1/2026

2,533
(2)
5,066

2.65

12/15/2026

8,066
(2)
18,335

2.70

1/17/2027

Manoj Maniar

5,625

—

57.60

3/16/2020

2,625

—

61.30

12/9/2020

378

—

61.30

12/4/2021

3,000

—

132.80

12/18/2022

500

—

150.00

7/25/2023

4,000
(1)
—

134.80

12/20/2023

4,500
(1)
1,500

39.80

12/17/2024

2,666
(1)
1,334

23.20

4/15/2025

2,250
(1)
1,750

14.80

9/24/2025

10,340

—

6.50

1/25/2026

7,083
(2)
9,917

3.24

9/1/2026

1,862
(2)
3,725

2.65

12/15/2026

5,931
(2)
13,482

2.70

1/17/2027

(1) Shares vest in equal monthly installments over four years, 1/48th per month. The first shares vest one month after the date of grant.
(2) Shares vest in equal monthly installments over three years, 1/36th per month. The first shares vest one month after the date of grant.
Potential Payments Upon Termination of Employment or Change in Control
As discussed under the caption “—Employment Agreements” above, we have agreements with our named executive officers pursuant to which they will receive severance payments upon certain termination events. The information below describes certain compensation that would be available under our existing plans and arrangements if (i) the named executive officer was terminated as of December 31, 2017 or (ii) if a Change in Control, as defined herein, occurred on December 31, 2017 and the named executive officer’s employment had been subsequently terminated on the same date.
Acceleration of Equity Awards
Pursuant to the terms of ~~such warrants, at no charge, non-transferable Subscription Rights to purchase one Unit at~~each named executive officer’s option agreements, in the event of a subscription price of $            per Unit. Each Basic Subscription Right will entitle you to purchase                share of our common stock and                Warrant for the purchase of one additional share of our common stock at an exercise price of $            per share from the date of issuance through its expiration on                . Each record holder will receive one Subscription Right for each whole share of our common stock owned by such record holder as of the Record Date. Each holder of an outstanding participating warrant will receive one Subscription Right for each whole share of our common stock as if each of such participating warrants had been exercised immediately“Change in Control” that occurs during any time prior to ~~the record date~~such named executive officer’s Termination of Service (as such terms are defined in our 2013 Equity Compensation Plan) with us, all stock options granted pursuant to such option agreement shall fully vest.
Termination Other than for ~~the Rights Offering. Each Subscription Right entitles the record holder~~Cause, Death or ~~holder of a participating warrant~~Disability; Resignation for Good Reason
The payments and benefits to ~~a Basic Subscription Right and an Over-Subscription Privilege.~~
~~Basic Subscription Rights~~
        Your Basic Subscription Rights will entitle you to purchase                share of our common stock and                Warrant to purchase one share of our common stock at the exercise price described elsewhere in this prospectus. For example, if you owned 100 shares of common stock as of the Record Date, you will receive 100 Subscription Rights and will have the right to purchase                shares of our common stock and Warrants to purchase                shares of our common stock for $            per whole Unit, or a total payment of $            . You may exercise all or a portion of your Basic Subscription Rights, or you may choose not to exercise any of your Basic Subscription Rights. If you do not exercise your Basic Subscription Rights in full, you will notwhich each named executive officer would be entitled ~~to exercise your Over-Subscription Privilege.~~
~~Over-Subscription Privilege~~
        If you exercise your Basic Subscription Rights in full, you may also choose to exercise your Over-Subscription Privilege. Subject to proration, if applicable, we will seek to honor the Over-Subscription Privilege requests in full. If Over-Subscription Privilege requests exceed the number of Units available, however, we will allocate the available Units pro rata among the record holders and participating warrant holders exercising the Over-Subscription Privilege in proportion to the number of shares of our common stock each of those record holders owned or the number of shares underlying participating warrants held by each of those warrant holders on the Record Date, relative to the number of shares owned or underlying participating warrants on the Record Date by all record holders and warrant holders exercising the Over-Subscription Privilege. If this pro rata allocation results in any record holder or warrant holder receiving a greater number of Units than the record holder or warrant holder subscribed for pursuant to the exercise of the Over-Subscription Privilege, then such record holder or warrant holder will be allocated only that number of Units for which the record holder or warrant holder oversubscribed, and the remaining Units will be allocated among all other record holders and warrant holders exercising the Over-Subscription Privilege on the same pro rata basis described above. The proration process will be repeated until all Units have been allocated.

~~Table of Contents~~
        ~~Wells Fargo Bank, N.A., the Subscription Agent for the Rights Offering, will determine the over-subscription allocation based on the formula described above.~~
        To the extent the aggregate subscription payment of the actual number of unsubscribed Units available to you pursuant to the Over-Subscription Privilege is less than the amount you actually paid in connection with the exercise of the Over-Subscription Privilege, you will be allocated only the number of unsubscribed Units available to you, and any excess subscription payments will be returned to you, without interest or penalty, as soon as practicable after expiration of the Rights Offering.
        We can provide no assurances that you will actually be entitled to purchase the number of Units issuable upon the exercise of your Over-Subscription Privilege in full at the expiration of the Rights Offering. We will not be able to satisfy any requests for Units pursuant to the Over-Subscription Privilege if all of our shareholders and participating warrant holders exercise their Basic Subscription Rights in full, and we will only honor an Over-Subscription Privilege to the extent sufficient Units are available following the exercise of Basic Subscription Rights.
~~Our Participating Warrant Holders~~
        On January 11, 2016, we issued common stock purchase warrants to purchase up to 96,842 shares of our common stock at an exercise price equal to $11.50 per share, subject to customary adjustments and as adjusted for our one-for-ten reverse stock split effective May 31, 2016. These warrants, referred to as our "participating warrants," entitle the holders to participate in this Rights Offering as if each of such participating warrants had been exercised immediately prior to the record date for the Rights Offering. As a result, holders of our participating warrants are receiving Subscription Rights for an aggregate of approximately 96,842 Units in connection with this Rights Offering; provided, however, to the extent that any warrant holder's right to participate in this Rights Offering would result in the ~~holder exceeding~~event the beneficial ownership limitation set forth in the participating warrants, then the holder will not be entitled to participate in this Rights Offering to such extent and the portion of this Rights Offering will be held in abeyance for the benefit of the holder until such time, if ever, as its right thereto would not result in the holder exceeding the applicable beneficial ownership limitation.
        ~~None of our other currently outstanding warrants is entitled to receive Subscription Rights in this offering.~~
~~Limitation on the Purchase of Units~~
        You may only purchase the number of whole Units purchasable upon exercise of the number of Basic Subscription Rights distributed to you in the Rights Offering, plus the Over-Subscription Privilege, if any. Accordingly, the number of Units that you may purchase in the Rights Offering is limited by the number of shares of our common stock you held on the Record Date or the number of shares underlying your outstanding participating warrants and by the extent to which other shareholders and participating warrant holders exercise their Basic Subscription Rights and Over-Subscription Privileges, which we cannot determine prior to completion of the Rights Offering.
~~Subscription Price~~
        The Subscription Price is $            per Unit. The Subscription Price does not necessarily bear any relationship to our past or expected future results of operations, cash flows, current financial condition, or any other established criteria for value. No change will be made to the Subscription Price by reason of changes in the trading price of our common stock or other factor prior to the expiration of this Rights Offering.

~~Table of Contents~~
~~Determination of Subscription Price~~
        ~~In the determining the Subscription Price, the board of directors considered a variety of factors including those listed below:~~
•
~~our need to raise capital in the near term to continue our operations;~~

•
~~the current and historical trading prices of our common stock;~~

•
~~a price that would increase the likelihood of participation in the Rights Offering;~~

•
~~the cost of capital from other sources;~~

•
~~the value of the Warrant being issued as a component of the Unit;~~

•
comparable precedent transactions, including the percentage of shares offered, the terms of the subscription rights being offered, the subscription price and the discount that the subscription price represents to the immediately prevailing closing prices for these offerings;

•
~~an analysis of stock price trading multiples for companies similar to us that, among other things, did not need to raise capital in the near-term; and~~

•
~~our most recently forecasted revenue relative to our peer group.~~
        The Subscription Price does not necessarily bear any relationship to any established criteria for value. No valuation consultant or investment banker has opined upon the fairness or adequacy of the Subscription Price. You should not consider the Subscription Price as an indication of actual value of our company or our common stock. You should not assume or expect that, after the Rights Offering, our shares of common stock will trade at or above the Subscription Price in any given time period. The market price of our common stock may decline during or after the Rights Offering. We cannot assure you that you will be able to sell the shares of our common stock purchased during the Rights Offering at a price equal to or greater than the Subscription Price. You should obtain a current price quote for our common stock before exercising your Subscription Rights and make your own assessment of our business and financial condition, our prospects for the future, and the terms of this Rights Offering. Once made, all exercises of Subscription Rights are irrevocable.
~~No Recombination~~
        The common stock and Warrants comprising the Units will separate upon the effectiveness of the exercise of the Subscription Rights and will be issued as separate securities, and the Units will not trade as a separate security. Holders may not recombine shares of common stock and Warrants to receive a Unit.
~~Non-Transferability of Subscription Rights~~
        The Subscription Rights are non-transferable (other than by operation of law) and, therefore, you may not sell, transfer, assign or give away your Subscription Rights to anyone. The Subscription Rights will not be listed for trading on any stock exchange or market.
~~Expiration Date; Extension~~
        The subscription period, during which you may exercise your Subscription Rights, expires at 5:00 PM Eastern Time, on                         , 2016, which is the expiration of the Rights Offering. If you do not exercise your Subscription Rights before that time, your Subscription Rights will expire and will no longer be exercisable. We will not be required to issue shares to you if the Subscription Agent receives your Subscription Rights Statement or your subscription payment after that time. We have the option to extend the Rights Offering in our sole discretion, although we do not presently intend to do

~~Table of Contents~~
so. We may extend the Rights Offering by giving oral or written notice to the Subscription Agent before the Rights Offering expires. If we elect to extend the Rights Offering, we will issue a press release announcing the extension no later than 9:00 AM Eastern Time, on the next business day after the most recently announced expiration date of the Rights Offering.
        If you hold your shares of common stock in the name of a broker, dealer, custodian bank or other nominee, the nominee will exercise the Subscription Rights on your behalf in accordance with your instructions. Please note that the nominee may establish a deadline that may be before 5:00 PM Eastern Time, on                        , 2016, which is the expiration date that we have established for the Rights Offering.
~~Termination~~
        We may terminate the Rights Offering at any time and for any reason prior to the completion of the Rights Offering. If we terminate the Rights Offering, we will issue a press release notifying shareholders, warrant holders and the public of the termination.
~~Return of Funds upon Completion or Termination~~
        The Subscription Agent will hold funds received in payment for shares in a segregated account pending completion of the Rights Offering. The Subscription Agent will hold this money until the Rights Offering is completed or is terminated. To the extent you properly exercise your Over-Subscription Privilege for an amount of Units that exceeds the number of unsubscribed Units available to you, any excess subscription payments will be returned to you as soon as practicable after the expiration of the Rights Offering, without interest or penalty. If the Rights Offeringnamed executive officer’s employment is terminated for any reason ~~all subscription payments received by the Subscription Agent will be returned as soon as practicable, without interest~~other than for cause, death, or ~~penalty.~~
~~Shares of Our Common Stock Outstanding After the Rights Offering~~
        After our one-for-ten reverse stock split on May 31, 2016, approximately 2,740,104 shares of our common stock were outstanding, along with participating warrants to purchase approximately 96,842 shares of common stock, in each case subject to final adjustments for treatment of fractional share interests. Based on the foregoing, and assuming no other transactions by us involving our common stock prior to the expiration of the Rights Offering,disability, or if the ~~Rights Offering~~named executive officer resigns for good reason, whether or not following a “change in control” is fully subscribed, approximately             shares of our common stock will be issued and outstanding and Warrants to purchase approximately            additional shares of our common stock will be outstanding (excluding the currently outstanding warrants). The exact number of shares and Warrants that we will issue in this Rights Offering will depend on the number of Units that are subscribed for in the Rights Offering.
~~Methods for Exercising Subscription Rights~~described above.
~~The exercise of Subscription Rights is irrevocable and may not be cancelled or modified. You may exercise your Subscription Rights as follows:~~
~~Subscription by Record Holders~~Equity Compensation Plan Information
If you are a shareholder of record or a holder of participating warrants, the number of Units you may purchase pursuant to your Subscription Rights in indicated on the enclosed Subscription Rights Statement. You may exercise your Subscription Rights by properly completing and executing the Subscription Rights Statement and forwarding it, together with your full payment, to the Subscription Agent at the address given below under "Subscription Agent," to be received before 5:00 PM Eastern Time, on                        , 2016.

~~Table of Contents~~
~~Subscription by Beneficial Owners~~
        If you are a beneficial owner of shares of our common stock that are registered in the name of a broker, dealer, custodian bank, or other nominee, you will not receive a Subscription Rights Statement. Instead, we will issue one Subscription Right to such nominee record holder for all shares of our common stock held by such nominee at the Record Date. If you are not contacted by your nominee, you should promptly contact your nominee in order to subscribe for shares in the Rights Offering and follow the instructions provided by your nominee.
        To properly exercise your Over-Subscription Privilege, you must deliver the subscription payment related to your Over-Subscription Privilege before the Rights Offering expires. Because we will not knowThe following table summarizes the total number of ~~unsubscribed Units before~~outstanding options and shares available for other future issuances of options under all of our equity compensation plans as of December 31, 2017. All of the ~~Rights Offering expires, if you wish to maximize~~outstanding awards listed below were granted under our 2013 Equity Compensation Plan. See “Stock Option and Other Compensation Plans—2013 Equity Compensation Plan” above for a summary of the 2013 Equity Compensation Plan.
Plan Category

Number of Shares to
be Issued Upon
Exercise of
Outstanding
Options,
Warrants and Rights

Weighted-Average
Exercise Price of
Outstanding
Options,
Warrants and Rights

Number of Shares
Remaining Available
for Future Issuance
Under the Equity
Compensation Plan
(Excluding Shares in
First Column)

Equity compensation plans approved by stockholders

894,996

$
40.41

57,632

Equity compensation plans not approved by stockholders

—

—

—

In accordance with the terms of the 2013 Equity Compensation Plan, on January 1, 2018, the maximum aggregate number of shares of Common Stock that may be issued under the plan was automatically increased by 200,000 shares, such that immediately after such increase the number of shares ~~you purchase pursuant to your Over-Subscription Privilege, you will need to deliver payment in an amount equal to the aggregate subscription payment~~remaining available for ~~the maximum number of Units that you wish to purchase.~~
~~Payment Method~~
        Payments must be made in full in U.S. currency by cashier's check or by wire transfer, and payable to "Wells Fargo Shareowner Services, as Subscription Agent for Onconova Therapeutics, Inc." You must timely pay the full subscription payment, including payment for the Over-Subscription Privilege, for the full number of Units of our common stock and Warrants you wish to acquire pursuant to the exercise of Subscription Rights by delivering a:
•
~~cashier's check, drawn on a U.S. bank payable to "Wells Fargo Shareowner Services, as Subscription Agent for Onconova Therapeutics, Inc."; or~~

•
wire transfer of immediately available funds directly to the account maintained by Wells Fargo Shareowner Services, as Subscription Agent, for purposes of accepting subscriptions in this Rights Offering at Wells Fargo Bank, N.A., ABA# 121000248, Credit: Stock Transfer Clearing Account # 000-10-67-899, for further credit to Onconova Therapeutics, Inc., and name of the Subscription Rights holder.
        ~~You should read the instruction letter accompanying the Subscription Rights Statement carefully and strictly follow it.DO NOT SEND SUBSCRIPTION RIGHTS STATEMENTS OR PAYMENTS DIRECTLY TO US~~. We will not consider your subscription received until the Subscription Agent has received delivery of a properly completed and duly executed Subscription Rights Statement and payment of the full subscription payment.
        The method of delivery of Subscription Rights Statements and payment of the subscription payment to the Subscription Agent will be at the risk of the holders of Subscription Rights. If sent by mail, we recommend that you send those statements and payments by registered mail, properly insured, with return receipt requested, or by overnight courier, and that you allow a sufficient number of days to ensure delivery to the Subscription Agent before the Rights Offering expires.
~~Missing or Incomplete Subscription Forms or Payment~~
        If you fail to complete and sign the Subscription Rights Statement or otherwise fail to follow the subscription procedures that apply to the exercise of your Subscription Rights before the Rights Offering expires, the Subscription Agent will reject your subscription or accept it to the extent of the payment received. Neither we nor our Subscription Agent undertakes any responsibility or action to contact you concerning an incomplete or incorrect subscription form, nor are we under any obligation to correct such forms. We have the sole discretion to determine whether a subscription exercise properly complies with the subscription procedures.

~~Table of Contents~~
        If you send a payment that is insufficient to purchase the number of shares you requested, or if the number of shares you requested is not specified in the forms, the payment received will be applied to exercise your Subscription Rights to the fullest extent possible based on the amount of the payment received. Any excess subscription payments received by the Subscription Agent will be returned, without interest or penalty, as soon as practicable following the expiration of the Rights Offering.
~~Issuance of common stock and Warrants~~
        The shares of common stock and Warrants that are purchased in the Rights Offering as part of the Units will be issued in book-entry, or uncertificated, form meaning that you will receive a direct registration (DRS) account statement from our transfer agent reflecting ownership of these securities if you are a holder of record of shares or warrants. If you hold your shares of common stock in the name of a custodian bank, broker, dealer, or other nominee, DTC will credit your account with your nominee with the securities you purchased in the Rights Offering.
~~Subscription Agent~~
        The Subscription Agent for the Rights Offering is Wells Fargo Bank, N.A. The address to which Subscription Rights Statements and payments should be mailed or delivered by overnight courier is provided below. If sent by mail, we recommend that you send documents and payments by registered mail, properly insured, with return receipt requested, and that you allow a sufficient number of days to ensure delivery to the Subscription Agent before the Rights Offering expires. Do not send or deliver these materials to us.
~~By Mail~~:
~~Wells Fargo Bank, N.A.~~
~~Shareowner Services~~
~~Voluntary Corporate Actions~~
~~P.O. Box 64858~~
~~St. Paul, Minnesota 55164-0858~~
~~By Hand or Overnight Courier~~:
~~Wells Fargo Bank, N.A.~~
~~Shareowner Services~~
~~Voluntary Corporate Actions~~
~~1110 Centre Pointe Curve, Suite 101~~
~~Mendota Heights, Minnesota 55120~~
        ~~If you deliver the Subscription Rights Statements in a manner different than that described in this prospectus, we may not honor the exercise of your Subscription Rights.~~
~~Dealer-Manager~~
        You should direct any questions or requests for assistance concerning the method of subscribing for the shares of our common stock or for additional copies of this prospectus to the dealer-manager for the Rights Offering as follows:
~~Maxim Group LLC~~
~~405 Lexington Avenue~~
~~New York, New York 10174~~
~~Attention Syndicate Department~~
~~Email: syndicate@maximgrp.com~~
~~Telephone: (212) 895-3745~~

~~Table of Contents~~
~~No Fractional Shares~~
        We will not issue fractional shares of common stock in the Rights Offering. Rights holders will only be entitled to purchase a number of Units representing a whole number of shares of common stock, rounded down to the nearest whole number of Units a holder would otherwise be entitled to purchase. Any excess subscription payments received by the Subscription Agent will be returned as soon as practicable after expiration of the Rights Offering, without interest or penalty. Similarly, no fractional shares of common stock will be issued in connection with the exercise of a Warrant. If, upon exercise of a Warrant, the holder thereof would be entitled to receive a fractional share of common stock, upon exercise, the holder will only be entitled to receive a whole number of shares of common stock, rounded down to the nearest whole number.
~~Notice to Brokers and Nominees~~
        If you are a broker, dealer, bank, or other nominee holder that holds shares of our common stock for the account of others on the Record Date, you should notify the beneficial owners of the shares for whom you are the nominee of the Rights Offering as soon as possible to learn their intentions with respect to exercising their Subscription Rights. If a beneficial owner of our common stock so instructs, you should complete the Subscription Rights Statement and submit it to the Subscription Agent with the proper subscription payment by the expiration date. You may exercise the number of Subscription Rights to which all beneficial owners in the aggregate otherwise would have been entitled had they been direct holders of our common stock on the Record Date, provided that you, as a nominee record holder, make a proper showing to the Subscription Agent by submitting the form entitled "Nominee Holder Certification," which is provided with your Rights Offering materials. If you did not receive this form, you should contact our Subscription Agent to request a copy.
~~Validity of Subscriptions~~
        We will resolve all questions regarding the validity and form of the exercise of your Subscription Rights, including time of receipt and eligibility to participate in the Rights Offering. Our determination will be final and binding. Once made, subscriptions are irrevocable; we will not accept any alternative, conditional, or contingent subscriptions. We reserve the absolute right to reject any subscriptions not properly submitted or the acceptance of which would be unlawful. You must resolve any irregularities in connection with your subscriptions before the expiration date of the Rights Offering, unless we waive them in our sole discretion. Neither we nor the Subscription Agent is under any duty to notify you or your representative of defects in your subscriptions. A subscription will be considered accepted, subject to our right to withdraw or terminate the Rights Offering, only when the Subscription Agent receives a properly completed and duly executed Subscription Rights Statement and any other required documents and the full subscription payment. Our interpretations of the terms and conditions of the Rights Offering will be final and binding.
~~Stockholder Rights~~
        You will have no rights as a holder of the shares of our common stock you purchase in the Rights Offering until shares are issued in book-entry form or your account at your broker, dealer, bank, or other nominee is credited with the shares of our common stock purchased in the Rights Offering. Holders of Warrants issued in connection with the Rights Offering will not have rights as holders of our common stock until such Warrants are exercised and the shares of common stock underlying the Warrants are issued to the holder.

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~~Foreign Shareholders~~
        We will not mail this prospectus or Subscription Rights Statements to shareholders with addresses that are outside the United States or that have an army post office or foreign post office address. The Subscription Agent will hold these Subscription Rights Statements for their account. To exercise Subscription Rights, our foreign shareholders must notify the Subscription Agent prior 5:00 PM Eastern Time, on                        , 2016, the third business day prior to the expiration date, of your exercise of Subscription Rights and provide evidence satisfactory to us, such as a legal opinion from local counsel, that the exercise of such Subscription Rights does not violate the laws of the jurisdiction in which such shareholder resides and payment by a U.S. bank in U.S. dollars before the expiration of the offer. If no notice is received by such time or the evidence presented is not satisfactory to us, the Subscription Rights represented thereby will expire.
~~No Revocation or Change~~
        Once you submit the Subscription Rights Statement or have instructed your nominee of your subscription request, you are not allowed to revoke or change the exercise or request a refund of monies paid. All exercises of Subscription Rights are irrevocable, even if you learn information about us that you consider to be unfavorable. You should not exercise your Subscription Rights unless you are certain that you wish to purchase shares at the Subscription Price.
~~U.S. Federal Income Tax Treatment of Rights Distribution~~
        For U.S. federal income tax purposes, we do not believe holders of shares of our common stock or warrants should recognize income or loss upon receipt or exercise of a Subscription Right. See "Material U.S. Federal Income Tax Consequences."
~~No Recommendation to Rights Holders~~
        Our board of directors is not making a recommendation regarding your exercise of the Subscription Rights. Stockholders who exercise Subscription Rights risk investment loss on money invested. We cannot assure you that the market price of our common stock will reach or exceed the Subscription Price, and even if it does so, that it will not decline during or after the Rights Offering. We also cannot assure you that you will be able to sell shares of our common stock or Warrants purchased in the Rights Offering at a price equal to or greater than the Subscription Price. You should make your investment decision based on your assessment of our business and financial condition, our prospects for the future ~~and the terms of this Rights Offering. Please see "Risk Factors" for a discussion of some of the risks involved in investing in our common stock.~~
~~Fees and Expenses~~
        We will pay all fees charged by the Subscription Agent and by the dealer-manager. You are responsible for paying any other commissions, fees, taxes or other expenses incurred in connection with the exercise of your Subscription Rights.
~~Listing~~
        The Subscription Rights may not be sold, transferred, assigned or given away to anyone, and will not be listed for trading on any stock exchange or market. We intend to apply to have the Warrants listed for trading on NASDAQissuance under the symbol "ONTXW," however, there is no assurance that a sufficient number of Subscription Rights will be exercised so that the Warrants will meet the minimum listing criteria to be accepted for listing on NASDAQ. The shares of our common stock, including the shares to be issued in the Rights Offering and the shares underlying the Warrants to be issued in the Rights Offering, are traded on NASDAQ under the symbol "ONTX."plan was 257,632.

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~~Important~~
Do not send Subscription Rights Statements directly to us. You are responsible for choosing the payment and delivery method for your Subscription Rights Statement and you bear the risks associated with such delivery. If you choose to deliver your Subscription Rights Statement and payment by mail, we recommend that you use registered mail, properly insured, with return receipt requested. We also recommend that you allow a sufficient number of days to ensure delivery to the Subscription Agent prior to the expiration time.
~~Distribution Arrangements~~
        Maxim Group LLC is the dealer-manager for the Rights Offering. The dealer-manager will provide marketing assistance and advice to us in connection with the Rights Offering and will use its best efforts to solicit the exercise of Subscription Rights and participation in the Over-Subscription Privilege. The dealer-manager is not underwriting or placing any of the Subscription Rights or the Units, shares of common stock or Warrants to be issued in the Rights Offering, and does not make any recommendation with respect to such Subscription Rights (including with respect to the exercise or expiration of such Subscription Rights), Units, shares of common stock or Warrants. We have agreed to pay the dealer-manager certain fees and to reimburse the dealer-manager for certain out-of-pocket expenses incurred in connection with this offering. See "Plan of Distribution" for a discussion of the fees and expenses to be paid to the dealer-manager in connection with this Rights Offering.

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~~MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES~~
        The following discussion is a summary of material U.S. federal income tax consequences relating to the receipt and exercise (or expiration) of the Subscription Rights acquired through the Rights Offering and the ownership and disposition of shares of our common stock and Warrants received upon exercise of the Subscription Rights or Warrants.
        This summary deals only with Subscription Rights acquired through the Rights Offering, shares of our common stock and Warrants acquired upon exercise of Subscription Rights and shares of our common stock acquired upon exercise of the Warrants, in each case, that are held as capital assets by a beneficial owner. This discussion does not address all aspects of U.S. federal income taxation that may be relevant to such a beneficial owner in light of their personal circumstances, including the alternative minimum tax and the Medicare contribution tax on investment income. This discussion also does not address tax consequences to holders that may be subject to special tax rules, including, without limitation, insurance companies, real estate investment trusts, regulated investment companies, grantor trusts, tax-exempt organizations, employee stock purchase plans, partnerships and other pass-through entities, persons holding Subscription Rights, shares of our common stock, participating warrants or Warrants as part of a hedging, integrated, conversion or constructive sale transaction or a straddle, financial institutions, brokers, dealers in securities or currencies, traders that elect to mark-to-market their securities, persons that acquired Subscription Rights, shares of our common stock, participating warrants or Warrants in connection with employment or other performance of services, U.S. Holders (as defined below) that have a functional currency other than the U.S. dollar, U.S. expatriates, and certain former citizens or residents of the United States. In addition, the discussion does not describe any tax consequences arising out of the tax laws of any state, local or foreign jurisdiction, or any U.S. federal tax considerations other than income taxation (such as estate, generation skipping or gift taxation).
        The discussion below is based upon the provisions of the Internal Revenue Code of 1986, as amended, or the Code, the United States Treasury regulations promulgated thereunder, rulings and judicial decisions, as of the date hereof, and such authorities may be repealed, revoked or modified, perhaps retroactively. We have not sought, and will not seek, any rulings from the Internal Revenue Service, or the IRS, regarding the matters discussed below. There can be no assurance that the IRS or a court (if the matter were contested) will not take positions concerning the tax consequences of the receipt of Subscription Rights acquired through the Rights Offering by persons holding shares of our common stock or participating warrants, the exercise (or expiration) of the Subscription Rights, the acquisition, ownership and disposition of shares of our common stock and the acquisition, ownership and disposition (or expiration) of Warrants acquired upon exercise of the Subscription Rights that are different from those discussed below.
        As used herein, a "U.S. Holder" means a beneficial owner of shares of our common stock, participating warrants, Subscription Rights, shares of our common stock and Warrants acquired upon exercise of Subscription Rights or shares of our common stock acquired upon exercise of Warrants, as the case may be, that is for U.S. federal income tax purposes: (1) an individual who is a citizen or resident of the United States; (2) a corporation (or other entity treated as a corporation for U.S. federal income tax purposes) created or organized in or under the laws of the United States or any state thereof or the District of Columbia; (3) an estate the income of which is subject to U.S. federal income taxation regardless of its source; or (4) a trust (a) the administration of which is subject to the primary supervision of a court within the United States and one or more United States persons as described in Section 7701(a)(30) of the Code have authority to control all substantial decisions of the trust or (b) that has a valid election under the Treasury Regulations in effect to be treated as a United States person. A "Non-U.S. Holder" is such a beneficial owner (other than an entity or arrangement that is treated as a partnership for U.S. federal income tax purposes) that is not a U.S. Holder.

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        If any entity or arrangement that is treated as a partnership for U.S. federal income tax purposes is the record owner, the U.S. federal income tax treatment of a partner generally will depend upon the status of the partner and the activities of the partnership. Holders that are partnerships (and partners in such partnerships) are urged to consult their own tax advisors.
        HOLDERS OF SHARES OF OUR COMMON STOCK AND PARTICIPATING WARRANTS SHOULD CONSULT THEIR OWN TAX ADVISORS REGARDING THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO THEIR PARTICULAR SITUATIONS AND THE CONSEQUENCES UNDER FEDERAL ESTATE AND GIFT TAX LAWS, FOREIGN, STATE AND LOCAL LAWS AND TAX TREATIES OF THE RECEIPT, OWNERSHIP AND EXERCISE OF SUBSCRIPTION RIGHTS AND THE ACQUISITION, OWNERSHIP AND DISPOSITION OF SHARES OF OUR COMMON STOCK AND WARRANTS ACQUIRED UPON EXERCISE OF SUBSCRIPTION RIGHTS AND SHARES OF OUR COMMON STOCK ACQUIRED UPON EXERCISE OF WARRANTS.
~~Tax Consequences to U.S. Holders~~
~~Taxation of Subscription Rights~~
~~Receipt of Subscription Rights~~
        Although the authorities governing transactions such as this Rights Offering are complex and do not speak directly to the consequences of certain aspects of this Rights Offering, including the inclusion of the right to purchase Warrants in the Subscription Rights (rather than the right to purchase only shares of our common stock), the distribution of Subscription Rights to participating warrant holders and the effects of the Over-Subscription Privilege, we do not believe your receipt of Subscription Rights pursuant to the Rights Offering should be treated as a taxable distribution with respect to your existing shares of common stock or participating warrants for U.S. federal income tax purposes. Pursuant to Section 305(a) of the Code, in general, the receipt by a shareholder or a warrant holder of a right to acquire stock or warrants should not be included in the taxable income of the recipient. The general rule of non-recognition in Section 305(a) is subject to exceptions in Section 305(b), which include "disproportionate distributions." A disproportionate distribution is a distribution or a series of distributions, including deemed distributions, that has the effect of the receipt of cash or other property by some shareholders and an increase in the proportionate interest of other shareholders in a corporation's assets or earnings and profits. During the last 36 months, we have not made any distributions of cash or non-stock property with respect to: (i) our common stock or (ii) our options or warrants to acquire common stock. Currently we do not intend to make any future distributions of cash or non-stock property with respect to: (i) our common stock or (ii) our options or warrants to acquire common stock; however, there is no guarantee that we will not make such distributions in the future.
        Our position regarding the tax-free treatment of the Subscription Rights distribution is not binding on the IRS or the courts. If this position is finally determined by the IRS or a court to be incorrect, whether on the basis that the issuance of the Subscription Rights is a "disproportionate distribution" or otherwise, the fair market value of the Subscription Rights would be taxable to holders of our common stock as a dividend to the extent of the holder's pro rata share of our current and accumulated earnings and profits, if any, with any excess being treated as a return of capital to the extent thereof and then as capital gain. Although no assurance can be given, it is anticipated that we will not have current and accumulated earnings and profits through the end of 2016. Further, if our position is incorrect, the treatment of holders of participating warrants may differ from the treatment of the Subscription Rights distribution to the holders of our common stock. The participating warrant holders may be treated in a manner similar to holders of our common stock but it is possible that they may be subject to different and adverse U.S. federal income tax consequences.

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        The following discussion is based upon the treatment of the Subscription Rights issuance as a non-taxable distribution with respect to your existing shares of common stock or participating warrants for U.S. federal income tax purposes.
~~Tax Basis in the Subscription Rights~~
        If the fair market value of the Subscription Rights you receive is less than 15% of the fair market value of your existing shares of common stock or participating warrants (with respect to which the Subscription Rights are distributed) on the date you receive the Subscription Rights, the Subscription Rights will be allocated a zero dollar basis for U.S. federal income tax purposes, unless you elect to allocate your basis in your existing shares of common stock or participating warrants between your existing shares of common stock or participating warrants and the Subscription Rights in proportion to the relative fair market values of the existing shares of common stock or participating warrants and the Subscription Rights, determined on the date of receipt of the Subscription Rights. If you choose to allocate basis between your existing common shares or participating warrants and the Subscription Rights, you must make this election on a statement included with your timely filed tax return (including extensions) for the taxable year in which you receive the Subscription Rights. Such an election is irrevocable.
        However, if the fair market value of the Subscription Rights you receive is 15% or more of the fair market value of your existing shares of common stock or participating warrants on the date you receive the Subscription Rights, then you must allocate your basis in your existing shares of common stock or participating warrants between those shares or participating warrants and the Subscription Rights you receive in proportion to their fair market values determined on the date you receive the Subscription Rights.
        The fair market value of the Subscription Rights on the date that the Subscription Rights are distributed is uncertain, and we have not obtained, and do not intend to obtain, an appraisal of the fair market value of the Subscription Rights on that date. In determining the fair market value of the Subscription Rights, you should consider all relevant facts and circumstances, including any difference between the Subscription Price of the Subscription Rights and the trading price of our shares of common stock on the date that the Subscription Rights are distributed, the exercise price of the Warrants, the length of the period during which the Subscription Rights may be exercised and the fact that the Subscription Rights are non-transferable.
~~Exercise of Subscription Rights~~
        Generally, you will not recognize gain or loss upon the effectiveness of the exercise of a Subscription Right in the Rights Offering. Your adjusted tax basis, if any, in the Subscription Right plus the Subscription Price should be allocated between the new common share and Warrant acquired upon exercise of the Subscription Right. The basis in the stock or participating warrants upon which the Subscriptions Rights were issued which is allocated to the Subscription Rights under the prior section entitled "Tax Basis in the Subscription Rights" would be further allocated between the new common share and the Warrant acquired upon exercise of the Subscription Right in proportion to their relative fair market values on the date the Subscription Rights were distributed. The Subscription Price should be allocated between the new common share and Warrant acquired upon exercise of the Subscription Right in proportion to their relative fair market values on the exercise date. These allocations will establish your initial tax basis for U.S. federal income tax purposes in your new common shares and Warrants. The holding period of a share of common stock or Warrant acquired upon exercise of a Subscription Right in the Rights Offering will begin on the date of exercise.
        If you exercise a Subscription Right received in the Rights Offering after disposing of the shares of our common stock or participating warrants with respect to which such Subscription Right is received,

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then certain aspects of the tax treatment of the exercise of the Subscription Right are unclear, including (1) the allocation of the tax basis between the shares of common stock or participating warrants previously sold and the Subscription Right, (2) the impact of such allocation on the amount and timing of gain or loss recognized with respect to the shares of our common stock or participating warrants previously sold and (3) the impact of such allocation on the tax basis of the shares of our common stock and Warrants acquired upon exercise of the Subscription Right. If you exercise a Subscription Right received in the Rights Offering after disposing of shares of our common stock or participating warrants with respect to which the Subscription Right is received, you should consult with your own tax advisor.
~~Expiration of Subscription Rights~~
        If you allow Subscription Rights received in the Rights Offering to expire, you should not recognize any gain or loss for U.S. federal income tax purposes, and you should re-allocate any portion of the tax basis in your existing common shares or participating warrants previously allocated to the Subscription Rights that have expired to the existing common shares or participating warrants.
~~Taxation of Warrants~~
~~Sale, Exchange, Redemption or other Taxable Disposition of Warrants~~
        Upon the sale, exchange, redemption or other taxable disposition of a Warrant, in general, you will recognize taxable gain or loss measured by the difference, if any, between (i) the amount of cash and the fair market value of any property received upon such taxable disposition and (ii) your adjusted tax basis in the Warrant as determined pursuant to the rules discussed above. Your gain or loss generally will be capital gain or loss and generally will be long-term capital gain or loss if, at the time of the sale or other disposition, your holding period for the Warrant is more than one year. The deductibility of capital losses is subject to limitations.
~~Exercise of Warrants~~
        Upon the exercise of a Warrant by paying the exercise price in cash, in general, you will not recognize gain or loss for U.S. federal income tax purposes, except to the extent you receive a cash payment for any such fractional share that would otherwise have been issuable upon exercise of the Warrant. Your initial tax basis in common stock received will equal your adjusted tax basis in the Warrant exercised (as determined pursuant to the rules discussed above), increased by the amount of cash paid to exercise the Warrant and decreased by the adjusted tax basis allocable to any fractional share that would otherwise have been issuable upon exercise of the Warrant. Your holding period for the shares of our common stock received on exercise generally will commence on the day of exercise.
        In certain circumstances, namely during any period when a registration statement for the exercise of the Warrants is not in effect, the Warrants will be exercisable on a cashless basis. The tax consequences of a cashless exercise are not clear and could differ from the consequences described above, including the possibility that a cashless exercise could be a taxable event. You should consult your own tax advisor regarding the tax consequences of a cashless exercise of a Warrant.
~~Expiration of Warrants~~
        If you allow a Warrant to expire, you will generally recognize a loss for U.S. federal income tax purposes equal to your adjusted tax basis in the Warrant. In general, such a loss will be a capital loss and will be a short-term or long-term capital loss depending on your holding period for the Warrant.

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~~Certain Adjustments to the Warrants~~
        Under Section 305 of the Code, an adjustment to the number of common shares that will be issued on the exercise of the Warrants, or an adjustment to the exercise price of the Warrants, may be treated as a constructive distribution to you if, and to the extent that, such adjustment has the effect of increasing your proportionate interest in our earnings and profits or assets, depending on the circumstances of such adjustment (for example, if such adjustment is to compensate for a distribution of cash or other property to our shareholders). Adjustments to the exercise price of Warrants made pursuant to a bona fide reasonable adjustment formula that has the effect of preventing dilution of the interest of the holders of the Warrants should generally not be considered to result in a constructive distribution. Any such constructive distribution would be taxable whether or not there is an actual distribution of cash or other property. See the more detailed discussion of the rules applicable to distributions made by us under the heading "Taxation of Common Shares—Distributions" below.
~~Taxation of Common Shares~~
~~Distributions~~
        Distributions with respect to shares of our common stock acquired upon exercise of Subscription Rights or upon exercise of Warrants will be taxable as dividend income when actually or constructively received to the extent of our current or accumulated earnings and profits as determined for U.S. federal income tax purposes.
        Dividend income received by certain non-corporate U.S. Holders with respect to shares of our common stock generally will be "qualified dividends" subject to preferential rates of U.S. federal income tax, provided that the U.S. Holder meets applicable holding period and other requirements. Subject to similar exceptions for short-term and hedged positions, dividend income on our shares of common stock paid to U.S. Holders that are domestic corporations generally will qualify for the dividends-received deduction. To the extent that the amount of a distribution exceeds our current and accumulated earnings and profits, such distribution will be treated first as a tax-free return of capital to the extent of your adjusted tax basis in such shares of our common stock and thereafter as capital gain.
~~Dispositions~~
        If you sell or otherwise dispose of shares of common stock acquired upon exercise of Subscription Rights or upon exercise of Warrants in a taxable transaction, you will generally recognize capital gain or loss equal to the difference between the amount realized and your adjusted tax basis in the shares. Such capital gain or loss will be long-term capital gain or loss if your holding period for such shares is more than one year at the time of disposition. Long-term capital gain of a non-corporate U.S. Holder is generally taxed at preferential rates of U.S. federal income tax. The deductibility of capital losses is subject to limitations.
~~Information Reporting and Backup Withholding~~
        You may be subject to information reporting and/or backup withholding with respect to the gross proceeds from the disposition of Warrants, shares of our common stock acquired through the exercise of Subscription Rights or through the exercise of Warrants, or dividend payments. Backup withholding (currently at the rate of 28%) may apply under certain circumstances if you (1) fail to furnish your social security or other taxpayer identification number, or TIN, (2) furnish an incorrect TIN, (3) fail to report interest or dividends properly or (4) fail to provide a certified statement, signed under penalty of perjury, that the TIN provided is correct, that you are not subject to backup withholding and that you are a U.S. person for U.S. federal income tax purposes on IRS Form W-9. Any amount withheld from a payment under the backup withholding rules is allowable as a credit against (and may entitle you to a refund with respect to) your U.S. federal income tax liability, provided that the required information is

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timely furnished to the IRS. Certain persons are exempt from information reporting and backup withholding, including corporations and certain financial institutions, provided that they demonstrate this fact, if requested. You are urged to consult your own tax advisor as to your qualification for exemption from backup withholding and the procedure for obtaining such exemption.
~~Tax Consequences to Non-U.S. Holders~~
~~Taxation of the Subscription Rights~~
~~Receipt, Exercise and Expiration of the Subscription Rights~~
        The discussion assumes that the receipt of Subscription Rights will be treated as a non-taxable distribution. See "Tax Consequences to U.S. Holders—Taxation of Subscription Rights—Receipt of Subscription Rights" above.
~~Exercise and Expiration of Warrants and Certain Adjustments to Warrants~~
~~Exercise of Warrants~~
        In general, a Non-U.S. Holder will not recognize gain or loss for U.S. federal income tax purposes upon exercise of a Warrant, except to the extent the Non-U.S. Holder receives a cash payment for any such fractional share that would otherwise have been issuable upon exercise of the Warrant, which will be treated as a sale subject to the rules described under "Sale or Other Disposition of Common Stock or Warrants" below.
~~Expiration of Warrants~~
        In general, a Non-U.S. Holder will not be able to utilize a loss recognized upon expiration of a Warrant against the Non-U.S. Holder's U.S. federal income tax liability unless the loss is effectively connected with the Non-U.S. Holder's conduct of a trade or business within the United States (and, if an income tax treaty so provides, is attributable to a permanent establishment in the United States) or is treated as a U.S.-source loss and the Non-U.S. Holder is present 183 days or more in the taxable year of disposition and certain other conditions are met.
~~Certain Adjustments to the Warrants~~
        Under Section 305 of the Code, an adjustment to the number of common shares that will be issued on the exercise of the Warrants, or an adjustment to the exercise price of the Warrants, may be treated as a constructive distribution to a Non-U.S. Holder of the Warrants if, and to the extent that, such adjustment has the effect of increasing such Non-U.S. Holder's proportionate interest in our "earnings and profits" or assets, depending on the circumstances of such adjustment (for example, if such adjustment is to compensate for a distribution of cash or other property to our shareholders). Adjustments to the exercise price of Warrants made pursuant to a bona fide reasonable adjustment formula that has the effect of preventing dilution of the interest of the holders of the Warrants should generally not be considered to result in a constructive distribution. Any such constructive distribution would be taxable whether or not there is an actual distribution of cash or other property. See the more detailed discussion of the rules applicable to distributions made by us under the heading "—Taxation of Distributions on Common Shares" below.
~~Taxation of Distributions on Common Shares~~
        Any distributions of cash or property (including any adjustments to the Warrants described in the immediately preceding paragraph) made with respect to our common stock generally will be subject to withholding tax to the extent paid out of our current or accumulated earnings and profits as determined for U.S. federal income tax purposes, if any, at a rate of 30% (or a lower rate prescribed

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by an applicable income tax treaty). In order to obtain a reduced withholding tax rate, if applicable, you will be required to provide a properly completed IRS Form W-8BEN or IRS Form W-8BEN-E, as applicable, certifying your entitlement to benefits under a treaty. In addition, you will not be subject to withholding tax if you provide an IRS Form W-8ECI certifying that the distributions are effectively connected with your conduct of a trade or business within the United States (and, if an applicable income tax treaty so provides, are attributable to a permanent establishment within the United States); instead, you generally will be subject to U.S. federal income tax, net of certain deductions, with respect to such income at the same rates applicable to U.S. persons. If you are a corporation, a "branch profits tax" of 30% (or a lower rate prescribed by an applicable income tax treaty) also may apply to such effectively connected income.
        ~~Non-U.S. Holders may be required to periodically update their IRS Forms W-8.~~
        ~~Any distribution will also be subject to the discussion below under the heading "FATCA."~~
~~Sale or Other Disposition of Our Common Stock or Warrants~~
        Subject to the discussion below regarding backup withholding and FATCA, you generally will not be subject to U.S. federal income tax on any gain realized on a sale or other disposition of shares of our common stock or Warrants unless:
•
the gain is effectively connected with your conduct of a trade or business within the United States (and, if an applicable income tax treaty so provides, is attributable to a permanent establishment in the United States);

•
you are an individual, you hold your Subscription Rights, shares of common stock or Warrants as capital assets, you are present in the United States for 183 days or more in the taxable year of disposition and certain other conditions are met (in which case you will be subject to a 30% tax, or such lower rate as may be specified by an applicable income tax treaty, on the net gain derived from the disposition, which may be offset by your U.S.-source capital losses, if any); or

•
~~we are or have been a "United States real property holding corporation," or USRPHC, for U.S. federal income tax purposes unless an exception for 5% or less shareholders applies.~~
        Gain that is effectively connected with your conduct of a trade or business within the United States (and, if an applicable income tax treaty so provides, is attributable to a permanent establishment within the United States) generally will be subject to U.S. federal income tax, net of certain deductions, at the same rates applicable to U.S. persons. If you are a corporation, a "branch profits tax" of 30% (or a lower rate prescribed in an applicable income tax treaty) also may apply to such effectively connected gain.
        A domestic corporation is treated as a USRPHC if the fair market value of its United States real property interests equals or exceeds 50% of the sum of (1) the fair market value of its United States real property interests, (2) the fair market value of its non-United States real property interests and (3) the fair market value of any other of its assets which are used or held for use in a trade or business. We believe that we are not currently, and have not been within the relevant testing period, a USRPHC. However, no assurance can be given that we will not become a USRPHC in the future. If we are a USRPHC or become a USRPHC in the future, a Non-U.S. Holder may still not be subject to U.S. federal income tax on a sale or other disposition if an exception for 5% or less shareholders applies. You are urged to consult your own tax advisor regarding the U.S. federal income tax considerations that could result if we are, or become, a USRPHC and with respect to the exception for 5% or less shareholders.

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~~Information Reporting and Backup Withholding~~
        Distributions on our common stock and the amount of tax withheld, if any, with respect to such distributions will generally be subject to information reporting. If you comply with certification procedures to establish that you are not a United States person, additional information reporting and backup withholding should not generally apply to distributions on our common stock and information reporting and backup withholding should not generally apply to the proceeds from a sale or other disposition of Warrants or shares of our common stock. Generally, a Non-U.S. Holder will comply with such procedures if it provides a properly executed IRS Form W-8BEN or W-8BEN-E, as applicable, (or other applicable IRS Form W-8) or otherwise meets documentary evidence requirements for establishing that it is a Non-U.S. Holder, or otherwise establishes an exemption. The amount of any backup withholding will generally be allowed as a refund or credit against your U.S. federal income tax liability, provided that the required information is timely furnished to the IRS.
~~FATCA~~
        Payments of dividends on our common shares to a Non-U.S. Holder will be subject to a 30% withholding tax if the Non-U.S. Holder fails to provide the withholding agent with documentation sufficient to show that it is compliant with FATCA. Generally such documentation is provided on an executed and properly completed IRS Form W-8BEN or IRS Form W-8BEN-E, as applicable. If dividends are subject to the 30% withholding tax under FATCA, they will not be subject to the 30% withholding tax described above under "Tax Consequences to Non-U.S. Holders—Taxation of Distributions on Common Shares." Starting in 2019, payments of the gross proceeds from a sale or exchange of our common shares or other securities may also be subject to FATCA withholding absent proof of FATCA compliance prior to January 1, 2019.
        THE PRECEDING DISCUSSION OF MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES IS NOT TAX ADVICE. HOLDERS OF SUBSCRIPTION RIGHTS, SHARES OF OUR COMMON STOCK AND PARTICIPATING WARRANTS SHOULD CONSULT THEIR OWN TAX ADVISORS REGARDING THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO THEIR PARTICULAR SITUATIONS AND THE CONSEQUENCES UNDER FEDERAL ESTATE AND GIFT TAX LAWS, FOREIGN, STATE AND LOCAL LAWS AND TAX TREATIES OF THE RECEIPT, OWNERSHIP AND EXERCISE OF SUBSCRIPTION RIGHTS AND THE ACQUISITION, OWNERSHIP AND DISPOSITION OF SHARES OF OUR COMMON STOCK AND WARRANTS ACQUIRED UPON EXERCISE OF SUBSCRIPTION RIGHTS AND SHARES OF OUR COMMON STOCK ACQUIRED UPON EXERCISE OF WARRANTS.

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DESCRIPTION OF ~~SECURITIES~~
CAPITAL STOCK

Our authorized capital stock consists of ~~25,000,000~~100,000,000 shares of ~~common stock,~~Common Stock, par value $0.01 per share, and 5,000,000 shares of preferred stock, par value $0.01 per share. ~~After our one-for-ten reverse stock split on May~~As of March 31, ~~2016, approximately 2,740,104~~2018, 19,426,163 shares of ~~our common stock were outstanding, subject to final adjustments for treatment of fractional share interests,~~Common Stock, and no shares of our preferred stock, were outstanding.
Common Stock

Subject to the preferences that may be applicable to any outstanding preferred stock, holders of ~~our common stock~~Common Stock are entitled to receive ratably any dividends that may be declared by our board of directors out of funds legally available for that purpose. Holders of ~~our common stock~~Common Stock are entitled to one vote for each share on all matters voted on by stockholders, including the election of directors. Holders of ~~our common stock~~Common Stock do not have any conversion, redemption, sinking fund or preemptive rights. In the event of our dissolution, liquidation or winding up, holders of ~~our common stock~~Common Stock are entitled to share ratably in any assets remaining after the satisfaction in full of the prior rights of creditors and the aggregate liquidation preference of any preferred stock then outstanding. The rights, preferences and privileges of the holders of ~~our common stock~~Common Stock are subject to, and may be adversely affected by, the rights of the holders of shares of any series of preferred stock that we may designate and issue in the future. All outstanding shares of ~~our common stock~~Common Stock are, and any shares of ~~common stock~~Common Stock that we may issue in the future will be, fully paid and non-assessable.
We do not currently have a sufficient number of authorized shares of Common Stock to cover the shares issuable upon the conversion of Series B Preferred Stock. As a result, before any shares of Series B Preferred Stock can become convertible, we need to receive stockholder approval of the Charter Amendment to sufficiently increase our authorized shares of Common Stock to cover the conversion of all outstanding shares of Series B Preferred Stock into Common Stock. We have agreed in the underwriting agreement for this offering to use our reasonable efforts to obtain such approval within 45 days from the date of this prospectus, and we intend to seek such approval at a special meeting of stockholders or at our 2018 annual meeting of stockholders. If approved by our stockholders, we intend to file the Charter Amendment with the Secretary of State of Delaware as soon as practicable following the special meeting or the annual meeting, as the case may be, and the Charter Amendment will be effective upon such filing.
We cannot assure you that we will be able to obtain requisite stockholder approval of the Charter Amendment. If the Charter Amendment is not approved by our stockholders, our Tenth Amended and Restated Certificate of Incorporation, as amended, will continue as currently in effect. In the event our stockholders do not approve the Charter Amendment, the Series B Preferred Stock will not be convertible into Common Stock and the value of the Warrants and the Series B Preferred Stock may be negatively affected.
Preferred Stock

We may issue any class of preferred stock in any series. Our board of directors has the authority, subject to limitations prescribed under Delaware law, to issue preferred stock in one or more series, to establish from time to time the number of shares to be included in each series and to fix the designation, powers, preferences and rights of the shares of each series and any of its qualifications, limitations and restrictions. Our board of directors can also increase or decrease the number of shares of any series, but not below the number of shares of that series then outstanding. Our board of directors may authorize the issuance of preferred stock with voting or conversion rights that could adversely affect the voting power or other rights of the holders of the ~~common stock.~~Common Stock. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions and other corporate purposes, could, among other things, have the effect of delaying, deferring or preventing a change in control of our company and may adversely affect the market price of ~~our common stock~~Common Stock and the voting and other rights of the holders of ~~common stock.~~
~~Warrants~~
~~Warrants Included in Units Issuable in the Rights Offering~~Common Stock.

Following this offering, we will have designated 1,300,000 shares of our preferred stock as Series B Convertible Preferred Stock. See “Description of Securities We Are Offering — Series B Convertible Preferred Stock” for a description of our Series B Convertible Preferred Stock.
Series A Convertible Preferred Stock
General
Our Board of Directors is authorized to issue up to 5,000,000 shares of preferred stock in one or more series without shareholder approval. Our Board of Directors may determine the designations, powers, preferences and the relative, participating, optional or other special rights, and any qualification, limitations and restrictions, of each series of preferred stock. Our Board of Directors has designated 1,044,488 shares of preferred stock as Series A Convertible Preferred Stock, which we refer to herein as the Series A Preferred Stock. As of March 31, 2018, there were no shares of preferred stock outstanding.
Rank
The ~~Warrants~~Series A Preferred Stock ranks (1) on parity with Common Stock on an “as converted” basis, (2) senior to any series of our capital stock hereafter created specifically ranking by its terms junior to the Series A Preferred Stock, (3) on parity with any series of our capital stock hereafter created specifically ranking by its terms on parity with the Series A Preferred Stock, and (4) junior to any series of our capital stock hereafter created specifically ranking by its terms senior to the Series A Preferred Stock in each case, as to dividends or distributions of assets upon our liquidation, dissolution or winding up whether voluntary or involuntary.
Conversion
Each 0.1 share of the Series A Preferred Stock is convertible into one (1) share of Common Stock, provided that the holder will be ~~issued~~prohibited from converting Series A Preferred Stock into shares of Common Stock if, as a ~~part~~ result
of ~~this Rights Offering will be separately transferable following their issuance and through their expiration            years from the date of issuance. The Warrants entitle~~such conversion, the holder ~~to purchase one share~~would own more than 4.99% of common stock at an exercise price of $            per share from the date of issuance through its expiration on                    . We intend to apply to list the Warrants for trading on NASDAQ under the symbol "ONTXW," however, there is no assurance that a sufficient number of Subscription Rights will be exercised so that the Warrants will meet the minimum listing criteria to be accepted for listing on NASDAQ. The common stock underlying the Warrants, upon issuance, will also be traded on NASDAQ under the symbol "ONTX."
        All Warrants that are purchased in the Rights Offering as part of the Units will be issued in book-entry, or uncertificated, form meaning that you will receive a direct registration (DRS) account statement from our transfer agent reflecting ownership of Warrants if you are a holder of record of

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shares or warrants. The Subscription Agent will arrange for the issuance of the Warrants as soon as practicable after the expiration of the Rights Offering, payment for the Units subscribed for has cleared, and all prorating calculations and reductions contemplated by the terms of the Rights Offering have been effected. If you hold your shares of common stock in the name of a custodian bank, broker, dealer, or other nominee, DTC will credit your account with your nominee with the Warrants you purchased in the Rights Offering.
        The Warrants will be exercisable by paying the exercise price in cash, or, solely during any period when a registration statement for the exercise of the Warrants is not in effect, exercisable on a cashless basis.
        ~~The exercise price of the Warrants and~~ the number of shares of ~~common stock~~Common Stock outstanding immediately after giving effect to the issuance of the shares of Common Stock issuable upon ~~exercise~~conversion of the ~~Warrants are~~Series A Preferred Stock, or, at the election of a holder, together with its affiliates, would own more than 9.99% of the number of shares of Common Stock outstanding immediately after giving effect to the issuance of the shares of Common Stock issuable upon conversion of the Series A Preferred Stock. The conversion rate of the Series A Preferred Stock is subject to proportionate adjustments for stock splits, reverse stock splits and similar events, but is not subject to adjustment based on price anti-dilution provisions.
Dividends
In addition to stock dividends or distributions for which proportionate adjustments will be made, holders of Series A Preferred Stock are entitled to receive dividends on shares of Series A Preferred Stock equal, on an as-if-converted-to-common-stock basis, to and in ~~certain circumstances, including a stock split of, stock dividend~~the same form as dividends actually paid on ~~or a subdivision, combination or recapitalization~~shares of the ~~common stock. Upon~~Common Stock when, as and if such dividends are paid on shares of the ~~merger, consolidation, sale~~Common Stock. No other dividends are payable on shares of ~~substantially all~~Series A Preferred Stock.
Voting Rights
Except as provided in the Certificate of ~~our assets,~~Designation or ~~other similar transaction,~~as otherwise required by law, the holders of ~~Warrants shall, at~~Series A Preferred Stock will have no voting rights. However, we may not, without the ~~option~~consent of holders of a majority of the ~~company, be required to exercise~~outstanding shares of Series A Preferred Stock, alter or change adversely the ~~Warrants immediately prior~~powers, preferences or rights given to the ~~closing~~Series A Preferred Stock, increase the number of authorized shares of Series A Preferred Stock, or enter into any agreement with respect to the foregoing.
Liquidation Rights
Upon any liquidation, dissolution or winding-up of the ~~transaction,~~Company, whether voluntary or ~~such Warrants shall automatically expire. Upon such exercise,~~involuntary, the holders of ~~Warrants shall participate on the same basis as~~Series A Preferred Stock are entitled to receive, pari passu with the holders of ~~common stock in connection with the transaction.~~
        ~~The Warrants do not confer upon the holder any voting or any other rights of a shareholder~~Common Stock, out of the ~~Company. Upon notice~~assets available for distribution to ~~the Warrants holders, we~~stockholders an amount equal to such amount per share as would have ~~the right at any time and from time to time, to reduce the exercise price or to extend the Warrants termination date.~~
        ~~The Warrants will be issued pursuant to a warrant agreement by and between us and Wells Fargo Bank, N.A., as the warrant agent.~~
~~Other Currently Outstanding Warrants~~
        ~~On January 11, 2016, we issued common stock purchase warrants, referred to as our "participating warrants," to purchase up to 96,842~~been payable had all shares of ~~our common stock at an exercise price equal~~Series A Preferred Stock been converted into Common Stock immediately before such liquidation, dissolution or winding up, without giving effect to ~~$11.50 per share, subject to customary adjustments~~any limitation on conversion as a result of the Beneficial Ownership Limitation, as described below.
Beneficial Ownership Limitation
We may not effect any conversion of the Series A Preferred Stock, and as adjusted for our one-for-ten reverse stock split effective May 31, 2016. Upon the terms and subject to the limitations on exercise and the conditions set forth in the participating warrants, the participating warrants are exercisable at any time on or after July 11, 2016 and on or prior to July 11, 2021. The participating warrants expire on July 11, 2021. The participating warrants entitle the holder to participate in any dividend or distribution, including any distribution of rights to purchase common stock, to the holders of our common stock. Subject to limited exceptions, a holder ~~of participating warrants will~~does not have the right to ~~exercise~~convert any portion of ~~its participating warrants if~~ the Series A Preferred Stock to the extent that, after giving effect to the conversion set forth in a notice of conversion such holder would beneficially own in excess of the Beneficial Ownership Limitation, or such holder, together with ~~its~~such holder’s affiliates, and any persons acting as a group together with such holder or affiliates, would beneficially own in excess of ~~9.99%~~the Beneficial Ownership Limitation. The “Beneficial Ownership Limitation” is 4.99% of the number of shares of ~~our common stock~~the Common Stock outstanding immediately after giving effect to ~~such exercise.~~the issuance of shares of Common Stock issuable upon conversion of Series A Preferred Stock held by the applicable holder. A holder may, with 61 days prior notice to us, elect to increase or decrease the Beneficial Ownership Limitation; provided, however, that in no event may either the holder Beneficial Ownership Limitation or the affiliate Beneficial Ownership Limitation be 9.99% or greater.
~~The participating warrants~~
Exchange Listing
Our Series A Preferred Stock is not listed on the Nasdaq Capital Market, any national securities exchange or other nationally recognized trading system. Our Common Stock issuable upon conversion of the Series A Preferred Stock is listed on the Nasdaq Capital Market.
Failure to Deliver Conversion Shares.
If we fail to timely deliver shares of Common Stock upon conversion of the Series A Preferred Stock (the “Conversion Shares”) within the time period specified in the Certificate of Designation (within three trading days after
delivery of the notice of conversion, or any shorter standard settlement period in effect with respect to trading market on the date notice is delivered), and if the holder has not exercised its Buy-In rights as described below with respect to such shares, then we are obligated to pay to the holder, as liquidated damages, an amount equal to $50 per trading day (increasing to $100 per trading day after the third trading day and $200 per trading day after the tenth trading day) for each $5,000 of Conversion Shares for which the Series A Preferred Stock converted which are not timely delivered. If we make such liquidated damages payments, we are not also obligated to make Buy-In payments with respect to the same Conversion Shares.
Compensation for Buy-In on Failure to Timely Deliver Shares
If we fail to timely deliver the Conversion Shares to the holder, and if after the required delivery date the holder is required by its broker to purchase (in an open market transaction or otherwise) or the holder or its brokerage firm otherwise purchases, shares of Common Stock to deliver in satisfaction of a sale by the holder of the Conversion Shares which the holder anticipated receiving upon such conversion or exercise (a “Buy-In”), then we are obligated to (A) pay in cash to the holder the amount, if any, by which (x) the holder’s total purchase price (including brokerage commissions, if any) for the shares of ~~our common~~Common Stock so purchased, minus any amounts paid to the holder by us as liquidated damages for late delivery of such shares, exceeds (y) the amount obtained by multiplying (1) the number of Conversion Shares that we were required to deliver times (2) the price at which the sell order giving rise to such purchase obligation was executed, and (B) at the option of the holder, either reinstate the portion of the Series A Preferred Stock and equivalent number of Conversion Shares for which such conversion was not honored (in which case such conversion shall be deemed rescinded) or deliver to the holder the number of shares of Common Stock that would have been issued had we timely complied with its conversion and delivery obligations.
Subsequent Rights Offerings; Pro Rata Distributions
If we grant, issue or sell any Common Stock equivalents pro rata to the record holders of any class of shares of Common Stock (the “Purchase Rights”), then a holder of Series A Preferred Stock will be entitled to acquire, upon the terms applicable to such Purchase Rights, the aggregate Purchase Rights which the holder could have acquired if the holder had held the number of shares of Common Stock acquirable upon conversion of the Series A Preferred Stock (without regard to any limitations on conversion). If we declare or make any dividend or other distribution of its assets (or rights to acquire its assets) to holders of Common Stock, then a holder of Series A Preferred Stock is entitled to participate in such distribution to the same extent as if the holder had held the number of shares of Common Stock acquirable upon complete conversion of the Series A Preferred Stock (without regard to any limitations on conversion).
Fundamental Transaction
If, at any time while the Series A Preferred Stock is outstanding, (i) the Company, directly or indirectly, in one or more related transactions effects any merger or consolidation of the Company with or into another person, (ii) the Company, directly or indirectly, effects any sale, lease, license, assignment, transfer, conveyance or other disposition of all or substantially all of its assets in one or a series of related transactions, (iii) any, direct or indirect, purchase offer, tender offer or exchange offer (whether by the Company or another person) is completed pursuant to which holders of Common Stock are permitted to sell, tender or exchange their shares for other securities, cash or property and has been accepted by the holders of 50% or more of the outstanding Common Stock, (iv) the Company, directly or indirectly, in one or more related transactions effects any reclassification, reorganization or recapitalization of the Common Stock or any compulsory share exchange pursuant to which the Common Stock is effectively converted into or exchanged for other securities, cash or property, or (v) the Company, directly or indirectly, in one or more related transactions consummates a stock or share purchase agreement or other business combination (including, without limitation, a reorganization, recapitalization, spin-off or scheme of arrangement) with another person whereby such other person acquires more than 50% of the outstanding shares of Common Stock (not including any shares of Common Stock held by the other person or other persons making or party to, or associated or affiliated with the other persons making or party to, such stock or share purchase agreement or other business combination) (each a “Series A Preferred Stock Fundamental Transaction”), then the Series A Preferred Stock automatically converts and the holder will receive, for each Conversion Share that would have been issuable upon ~~exercise~~such conversion immediately prior to the occurrence of such Series A Preferred Stock Fundamental Transaction (without regard to the Beneficial Ownership Limitation), the number of shares of Common Stock of the ~~participating warrants were offered~~successor or acquiring corporation or of the Company, if it
is the surviving corporation, and ~~sold without registration under~~any additional consideration (the “Series A Preferred Stock Alternate Consideration”) receivable as a result of such Series A Preferred Stock Fundamental Transaction by a holder of the ~~Securities Act~~number of ~~1933, as amended ("~~shares of Common Stock for which the ~~Securities Act"), or state securities laws, in reliance~~Series A Preferred Stock is convertible immediately prior to such Series A Preferred Stock Fundamental Transaction (without regard to the Beneficial Ownership Limitation). For purposes of any such conversion, the determination of the conversion ratio will be appropriately adjusted to apply to such Series A Preferred Stock Alternate Consideration based on the ~~exemptions provided by Section 4(a)(2)~~amount of Alternate Consideration issuable in respect of one share of Common Stock in such Series A Preferred Stock Fundamental Transaction. If holders of Common Stock are given any choice as to the securities, cash or property to be received in a Series A Preferred Stock Fundamental Transaction, then the holder will be given the same choice as to the Series A Preferred Stock Alternate Consideration it receives upon automatic conversion of the Act and/or Regulation D promulgated thereunder and in reliance on similar exemptions under applicable state laws. Accordingly, neither the participating warrants nor the shares of our common stock underlying the participating warrants may be offered or sold except pursuant to an effective registration statement under the Securities Act or pursuant to an available exemption from, or in a transaction not subject to, the registration requirements of the Securities Act and in accordance with applicable state securities laws.Series A Preferred Stock following such Fundamental Transaction.
~~Prior to our initial public offering, in connection with a credit facility, we issued a warrant to purchase 6,128 shares of Series G convertible preferred stock in June 2009. The warrant was~~

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immediately exercisable upon issuance and expires on July 30, 2016. Following the consummation of our initial public offering and the conversion of our Series G convertible preferred stock into shares of common stock in July 2013, and subsequent adjustments pursuant to the adjustment provisions set forth in that warrant, after our one-for-ten reverse stock split on May 31, 2016, the warrant was exercisable for approximately 582 shares of common stock at an exercise price per share of approximately $103.08.
Delaware Anti-Takeover Law and Provisions in Our Certificate of Incorporation and Bylaws
Delaware Anti-Takeover Law

We are subject to Section 203 of the Delaware General Corporation Law. Section 203 generally prohibits a public Delaware corporation from engaging in a ~~"business combination"~~“business combination” with an ~~"interested stockholder"~~“interested stockholder” for a period of three years after the date of the transaction in which the person became an interested stockholder, unless:
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·prior to the date of the transaction, the board of directors of the corporation approved either the business combination or the transaction which resulted in the stockholder becoming an interested stockholder;
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upon consummation of the transaction that resulted in the stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding specified shares; or
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at or subsequent to the date of the transaction, the business combination is approved by the board of directors and authorized at an annual or special meeting of stockholders, and not by written consent, by the affirmative vote of at least 66²/3%662/3% of the outstanding voting stock which is not owned by the interested stockholder.

Section 203 defines a ~~"business combination"~~“business combination” to include:
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·any merger or consolidation involving the corporation and the interested stockholder;
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any sale, lease, exchange, mortgage, pledge, transfer or other disposition of 10% or more of the assets of the corporation to or with the interested stockholder;
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subject to exceptions, any transaction that results in the issuance or transfer by the corporation of any stock of the corporation to the interested stockholder;
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subject to exceptions, any transaction involving the corporation that has the effect of increasing the proportionate share of the stock of any class or series of the corporation beneficially owned by the interested stockholder; or
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the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits provided by or through the corporation.

In general, Section 203 defines an ~~"interested stockholder"~~“interested stockholder” as any person that is:
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·the owner of 15% or more of the outstanding voting stock of the corporation;
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an      stock of the corporation at any time within three years immediately prior to the affiliate or associate of the corporation who was the owner of 15% or more of the outstanding voting ~~stock of the corporation at any time within three years immediately prior to the~~ relevant date; or
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the affiliates and associates of the above.

Under specific circumstances, Section 203 makes it more difficult for an ~~"interested stockholder"~~“interested stockholder” to effect various business combinations with a corporation for a three-year period, although the stockholders may, by adopting an amendment to the ~~corporation's~~corporation’s certificate of incorporation or bylaws, elect not to be governed by this section, effective 12 months after adoption.

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Our certificate of incorporation and bylaws do not exclude us from the restrictions of Section 203. We anticipate that the provisions of Section 203 might encourage companies interested in acquiring us to negotiate in advance with our board of directors since the stockholder approval requirement would be avoided if a majority of the directors then in office approve either the business combination or the transaction that resulted in the stockholder becoming an interested stockholder.
Certificate of Incorporation and Bylaws

Provisions of our certificate of incorporation and bylaws may delay or discourage transactions involving an actual or potential change of control or change in our management, including transactions in which stockholders might otherwise receive a premium for their shares, or transactions that our stockholders might otherwise deem to be in their best interests. Therefore, these provisions could adversely affect the price of ~~our common stock.~~Common Stock. Among other things, our certificate of incorporation and bylaws will:
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·permit our board of directors to issue up to 5,000,000 shares of preferred stock, with any rights, preferences and privileges as they may designate;
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provide that all vacancies on our board of directors, including as a result of newly created directorships, may, except as otherwise required by law, be filled by the affirmative vote of a majority of directors then in office, even if less than a quorum;
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require that any action to be taken by our stockholders must be effected at a duly called annual or special meeting of stockholders and not be taken by written consent;
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provide that stockholders seeking to present proposals before a meeting of stockholders or to nominate candidates for election as directors at a meeting of stockholders must provide advance notice in writing, and also specify requirements as to the form and content of a ~~stockholder's~~stockholder’s notice;
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not provide for cumulative voting rights, thereby allowing the holders of a majority of the shares of ~~common stock~~Common Stock entitled to vote in any election of directors to elect all of the directors standing for election; and
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provide that special meetings of our stockholders may be called only by the board of directors or by such person or persons requested by a majority of the board of directors to call such meetings.
Transfer Agent

The transfer agent and registrar for ~~our common stock~~Common Stock is ~~Wells Fargo Bank, N.A.~~EQ Shareowner Services.
Listing

Our ~~common stock~~Common Stock is listed on the Nasdaq Capital Market under the symbol ~~"ONTX."~~“ONTX.”

~~Table~~DESCRIPTION OF SECURITIES WE ARE OFFERING
We are offering (i) up to 26,785,714 Units, each Unit consisting of ~~Contents~~one share of Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock, or (ii) up to 26,785,714 Pre-Funded Units, each Pre-Funded Unit consisting of one Pre-Funded Warrant to purchase one share of Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock. For each Pre-Funded Unit we sell, the number of Units we are offering will be decreased on a one-for-one basis. The share of Common Stock and accompanying Warrant included in each Unit will be issued separately, and the Pre-Funded Warrant to purchase one share of Common Stock and the accompanying Warrant included in each Pre-Funded Unit will be issued separately. Units or Pre-Funded Units will not be issued or certificated. We are also registering the shares of Common Stock included in the Units and the shares of Common Stock issuable from time to time upon exercise of the Pre-Funded Warrants included in the Pre-Funded Units, the Warrants included in the Units and the Pre-Funded Units offered hereby, the Pre-Funded Warrants included in the Pre-Funded Units and the shares of Series B Preferred Stock issuable upon conversion of the Warrants.
Common Stock
The material terms and provisions of Common Stock and each other class of our securities which qualifies or limits Common Stock are described under the caption “Description of Capital Stock” in this prospectus.
Series B Convertible Preferred Stock

General
Our Board of Directors is authorized to issue up to 5,000,000 shares of preferred stock in one or more series without shareholder approval. Our Board of Directors may determine the designations, powers, preferences and the relative, participating, optional or other special rights, and any qualification, limitations and restrictions, of each series of preferred stock. In February 2018 our Board of Directors designated 1,044,488 shares of preferred stock as Series A Convertible Preferred Stock. As of April 20, 2018, there were no shares of preferred stock outstanding.
Our Board of Directors has designated 1,300,000 shares of preferred stock as Series B Convertible Preferred Stock, which we refer to herein as the Series B Preferred Stock.
Rank
The Series B Preferred Stock ranks (1) on parity with Common Stock on an “as converted” basis, (2) senior to any series of our capital stock hereafter created specifically ranking by its terms junior to the Series B Preferred Stock, (3) on parity with Series A Preferred Stock and any series of our capital stock hereafter created specifically ranking by its terms on parity with the Series B Preferred Stock, and (4) junior to any series of our capital stock hereafter created specifically ranking by its terms senior to the Series B Preferred Stock in each case, as to dividends or distributions of assets upon our liquidation, dissolution or winding up whether voluntary or involuntary.
Conversion

~~PLAN OF DISTRIBUTION~~
        ~~On or about                , 2016,~~We do not currently have a sufficient number of authorized shares of Common Stock to cover the shares issuable upon the conversion of Series B Preferred Stock. As a result, before any shares of Series B Preferred Stock can become convertible, we ~~will distribute~~need to receive stockholder approval of the ~~Subscription Rights, Subscription Rights Statements and copies~~Charter Amendment to sufficiently increase our authorized shares of Common Stock to cover the conversion of all outstanding shares of Series B Preferred Stock into Common Stock. We have agreed in the underwriting agreement for this offering to use our reasonable efforts to obtain such approval within 45 days from the date of this prospectus, and we intend to seek such approval at a special meeting of stockholders or at our 2018 annual meeting of stockholders. We cannot assure you that we will be able to obtain requisite stockholder approval of the Charter Amendment. Series B Preferred Stock is not convertible until the next business day after the Charter Amendment Date (which is the date on which we publicly announce through the filing of a Current Report on Form 8-K that the Charter Amendment has been filed with the Secretary of State of the State of Delaware), starting at which time each 0.025 share of the Series B Preferred Stock is convertible into one (1) share of Common Stock, provided that the holder will be prohibited from converting Series B Preferred Stock into shares of Common Stock if, as a result of such conversion, the holder would own more than 4.99% of the number of shares of Common Stock outstanding immediately after giving effect to the issuance of the shares of Common Stock issuable upon conversion of the Series B Preferred Stock, or, at the election of a holder, together with its affiliates, would own more than 9.99% of the number of shares of Common Stock outstanding immediately after giving effect to the issuance of the shares of Common Stock issuable upon conversion of the Series B Preferred Stock. The conversion rate of the Series B Preferred Stock is subject to proportionate adjustments for stock splits, reverse stock splits and similar events, but is not subject to adjustment based on price anti-dilution provisions. In the event our stockholders do not approve the Charter Amendment, the Series B Preferred Stock will not be convertible into Common Stock and the value of Series B Preferred Stock may be negatively affected.
Dividends
In addition to stock dividends or distributions for which proportionate adjustments will be made, holders of Series B Preferred Stock are entitled to receive dividends on shares of Series B Preferred Stock equal, on an as-if-converted-to-common-stock basis, to and in the same form as dividends actually paid on shares of the Common Stock when, as and if such dividends are paid on shares of the Common Stock. No other dividends are payable on shares of Series B Preferred Stock.
Voting Rights
Except as provided in the Certificate of Designation or as otherwise required by law, the holders of ~~our common stock~~Series B Preferred Stock will have no voting rights. However, we may not, without the consent of holders of a majority of the outstanding shares of Series B Preferred Stock, alter or change adversely the powers, preferences or rights given to the Series B Preferred Stock, increase the number of authorized shares of Series B Preferred Stock, or enter into any agreement with respect to the foregoing.
Liquidation Rights
Upon any liquidation, dissolution or winding-up of the Company, whether voluntary or involuntary, the holders of Series B Preferred Stock are entitled to receive, pari passu with the holders of Common Stock, out of the assets available for distribution to stockholders an amount equal to such amount per share as would have been payable had all shares of Series B Preferred Stock been converted into Common Stock immediately before such liquidation, dissolution or winding up, without giving effect to any limitation on conversion as a result of the Beneficial Ownership Limitation, as described below.
Beneficial Ownership Limitation
We may not effect any conversion of the Series B Preferred Stock, and ~~participating warrants~~a holder does not have the right to convert any portion of the Series B Preferred Stock to the extent that, after giving effect to the conversion set forth in a notice of conversion such holder would beneficially own in excess of the Beneficial Ownership Limitation, or such holder, together with such holder’s affiliates, and any persons acting as a group together with such holder or affiliates, would beneficially own in excess of the Beneficial Ownership Limitation. The “Beneficial Ownership Limitation” is 4.99% of the number of shares of the Common Stock outstanding immediately after giving effect to the issuance of shares of Common Stock issuable upon conversion of Series B Preferred Stock held by the applicable holder. A holder may, with 61 days prior notice to us, elect to increase or decrease the Beneficial Ownership Limitation; provided, however, that in no event may either the holder Beneficial Ownership Limitation or the affiliate Beneficial Ownership Limitation be 9.99% or greater.
Exchange Listing
We do not plan on making an application to list the shares of Series B Preferred Stock on the ~~Record Date. Subscription Rights holders who wish~~Nasdaq Capital Market, any national securities exchange or other nationally recognized trading system. Our Common Stock issuable upon conversion of the Series B Preferred Stock is listed on the Nasdaq Capital Market.
Failure to Deliver Conversion Shares.
If we fail to timely deliver shares of Common Stock upon conversion of the Series B Preferred Stock (the “Conversion Shares”) within the time period specified in the Certificate of Designation (within three trading days after delivery of the notice of conversion, or any shorter standard settlement period in effect with respect to trading market on the date notice is delivered), and if the holder has not exercised its Buy-In rights as described below with respect to such shares, then we are obligated to pay to the holder, as liquidated damages, an amount equal to $50 per trading day (increasing to $100 per trading day after the third trading day and $200 per trading day after the tenth trading day) for each $5,000 of Conversion Shares for which the Series B Preferred Stock converted which are not timely delivered. If we make such liquidated damages payments, we are not also obligated to make Buy-In payments with respect to the same Conversion Shares.
Compensation for Buy-In on Failure to Timely Deliver Shares
If we fail to timely deliver the Conversion Shares to the holder, and if after the required delivery date the holder is required by its broker to purchase (in an open market transaction or otherwise) or the holder or its brokerage firm otherwise purchases, shares of Common Stock to deliver in satisfaction of a sale by the holder of the Conversion Shares which the holder anticipated receiving upon such conversion or exercise ~~their Subscription Rights and~~(a “Buy-In”), then we are obligated to (A) pay in cash to the holder the amount, if any, by which (x) the holder’s total purchase ~~Units must complete the Subscription Rights Statement and return it with payment~~price (including brokerage commissions, if any) for the shares of Common Stock so purchased, minus any amounts paid to the ~~Subscription Agent~~holder by us as liquidated damages for late delivery of such shares, exceeds (y) the amount obtained by multiplying (1) the number of Conversion Shares that we were required to deliver times (2) the price at which the
sell order giving rise to such purchase obligation was executed, and (B) at the ~~following address:~~
~~By Mail~~:
~~Wells Fargo Bank, N.A.~~
~~Shareowner Services~~
~~Voluntary Corporate Actions~~
~~P.O. Box 64858~~
~~St. Paul, Minnesota 55164-0858~~
~~By Hand~~option of the holder, either reinstate the portion of the Series B Preferred Stock and equivalent number of Conversion Shares for which such conversion was not honored (in which case such conversion shall be deemed rescinded) or ~~Overnight Courier~~:
~~Wells Fargo Bank, N.A.~~
~~Shareowner Services~~
~~Voluntary Corporate Actions~~
~~1110 Centre Pointe Curve, Suite 101~~
~~Mendota Heights, Minnesota 55120~~deliver to the holder the number of shares of Common Stock that would have been issued had we timely complied with its conversion and delivery obligations.
~~See "The~~
Subsequent Rights ~~Offering—Methods~~Offerings; Pro Rata Distributions
If we grant, issue or sell any Common Stock equivalents pro rata to the record holders of any class of shares of Common Stock (the “Purchase Rights”), then a holder of Series B Preferred Stock will be entitled to acquire, upon the terms applicable to such Purchase Rights, the aggregate Purchase Rights which the holder could have acquired if the holder had held the number of shares of Common Stock acquirable upon conversion of the Series B Preferred Stock (without regard to any limitations on conversion). If we declare or make any dividend or other distribution of its assets (or rights to acquire its assets) to holders of Common Stock, then a holder of Series B Preferred Stock is entitled to participate in such distribution to the same extent as if the holder had held the number of shares of Common Stock acquirable upon complete conversion of the Series B Preferred Stock (without regard to any limitations on conversion).
Fundamental Transaction
If, at any time while the Series B Preferred Stock is outstanding, (i) the Company, directly or indirectly, in one or more related transactions effects any merger or consolidation of the Company with or into another person, (ii) the Company, directly or indirectly, effects any sale, lease, license, assignment, transfer, conveyance or other disposition of all or substantially all of its assets in one or a series of related transactions, (iii) any, direct or indirect, purchase offer, tender offer or exchange offer (whether by the Company or another person) is completed pursuant to which holders of Common Stock are permitted to sell, tender or exchange their shares for ~~Exercising Subscription Rights."~~other securities, cash or property and has been accepted by the holders of 50% or more of the outstanding Common Stock, (iv) the Company, directly or indirectly, in one or more related transactions effects any reclassification, reorganization or recapitalization of the Common Stock or any compulsory share exchange pursuant to which the Common Stock is effectively converted into or exchanged for other securities, cash or property, or (v) the Company, directly or indirectly, in one or more related transactions consummates a stock or share purchase agreement or other business combination (including, without limitation, a reorganization, recapitalization, spin-off or scheme of arrangement) with another person whereby such other person acquires more than 50% of the outstanding shares of Common Stock (not including any shares of Common Stock held by the other person or other persons making or party to, or associated or affiliated with the other persons making or party to, such stock or share purchase agreement or other business combination) (each a “Series B Preferred Stock Fundamental Transaction”), then the Series B Preferred Stock automatically converts and the holder will receive, for each Conversion Share that would have been issuable upon such conversion immediately prior to the occurrence of such Series B Preferred Stock Fundamental Transaction (without regard to the Beneficial Ownership Limitation), the number of shares of Common Stock of the successor or acquiring corporation or of the Company, if it is the surviving corporation, and any additional consideration (the “Series B Preferred Stock Alternate Consideration”) receivable as a result of such Series B Preferred Stock Fundamental Transaction by a holder of the number of shares of Common Stock for which the Series B Preferred Stock is convertible immediately prior to such Series B Preferred Stock Fundamental Transaction (without regard to the Beneficial Ownership Limitation). For purposes of any such conversion, the determination of the conversion ratio will be appropriately adjusted to apply to such Series B Preferred Stock Alternate Consideration based on the amount of Alternate Consideration issuable in respect of one share of Common Stock in such Series B Preferred Stock Fundamental Transaction. If holders of Common Stock are given any choice as to the securities, cash or property to be received in a Series B Preferred Stock Fundamental Transaction, then the holder will be given the same choice as to the Series B Preferred Stock Alternate Consideration it receives upon automatic conversion of the Series B Preferred Stock following such Fundamental Transaction.
The Warrants
~~If you~~
The following is a summary of all material terms and provisions of the Warrants that are being offered hereby, the form of which has been filed as an exhibit to the registration statement of which this prospectus is a part. Prospective investors should carefully review the terms and provisions of the form of Warrant for a complete description of the terms and conditions of the Warrants.
Duration and Exercise Price
Each Warrant offered hereby will have ~~any questions, you should contact~~an exercise price equal to $     per 0.025 share of Series B Preferred Stock. The Warrants will be immediately exercisable and may be exercised until the ~~dealer-manager~~18-month anniversary of the Charter Amendment Date, at which time they will be automatically exercised on a cashless basis. The exercise price and number of shares of Series B Preferred Stock issuable upon exercise is subject to appropriate adjustment in the event of stock dividends, stock splits, reorganizations or similar events affecting Series B Preferred Stock and the exercise price. The Warrants will be issued separately from the Common Stock or Pre-Funded Warrants sold as part of the Units or Pre-Funded Units, respectively, and may be transferred separately immediately thereafter. Warrants will be issued in certificated form only.
Exercisability
The Warrants will be exercisable, at the option of each holder, in whole or in part, by delivering to us a duly executed exercise notice accompanied by payment in full for the ~~Rights Offering:~~
~~Maxim Group LLC~~
~~405 Lexington Avenue~~
~~New York, New York 10174~~
~~Attention Syndicate Department~~
~~Email: syndicate@maximgrp.com~~
~~Telephone: (212) 895-3745~~number of shares of Series B Preferred Stock purchased upon such exercise (except in the case of a cashless exercise as discussed below).
~~Other than~~
Cashless Exercise
If, at the time a holder exercises its Warrants, a registration statement registering the issuance of the shares of Series B Preferred Stock underlying the Warrants under the Securities Act is not then effective or available for the issuance of such shares, then in lieu of making the cash payment otherwise contemplated to be made to us upon such exercise in payment of the aggregate exercise price, the holder may elect instead to receive upon such exercise (either in whole or in part) the net number of shares of Series B Preferred Stock determined according to a formula set forth in the Warrant. The Warrants will be automatically exercised on a cashless basis on the expiration date.
Fundamental Transactions
In the event of any fundamental transaction, as described in ~~this prospectus, we do not know~~the Warrants and generally including any merger with or into another entity, sale of all or substantially all of our assets, tender offer or exchange offer, or reclassification of Common Stock, then upon any ~~existing agreements between~~subsequent exercise of a Warrant, the holder will have the right to receive as alternative consideration, for each share of Common Stock that the holder would have received upon such holder’s exercise of the Warrant into shares of Series B Preferred Stock and the conversion of such shares of Series B Preferred Stock to shares of Common Stock (without giving effect to any ~~shareholder, broker, dealer, underwriter or agent relating~~limitation on conversion as a result of the Series B Preferred Stock Beneficial Ownership Limitation) immediately prior to the ~~sale or distribution~~occurrence of such fundamental transaction, the ~~underlying common stock.~~
        ~~Maxim Group LLC is the dealer-manager~~number of this Rights Offering. We and Maxim may introduce one or more co-dealer-managers and one or more financial advisors to assist in the Rights Offering. In any such event, Maxim Group LLC will be the lead dealer-manager. In such capacity, the dealer-manager will provide marketing assistance and advice to us in connection with this offering and will solicit the exercise of Subscription Rights and participation in the Over-Subscription Privilege. The dealer-manager is not underwriting or placing any of the Subscription Rights or the Units, shares of common stock of the successor or acquiring corporation or of our company, if it is the surviving corporation, and any additional consideration receivable upon or as a result of such transaction by a holder of the number of shares of Common Stock for which the holder would have received upon such holder’s exercise of the Warrant into shares of Series B Preferred Stock and the conversion of such shares of Series B Preferred Stock to shares of Common Stock (without giving effect to any limitation on conversion as a result of the Series B Preferred Stock Beneficial Ownership Limitation) immediately prior to the occurrence of such fundamental transaction.
Transferability
Subject to applicable laws and a standard legend with regard to restriction on transfer only in compliance with a public offering or an available exemption therefrom, the Warrant may be transferred at the option of the holder upon surrender of the Warrant to us together with the appropriate instruments of transfer.
No Listing
There is no established trading market for the Warrants, and we do not expect an active trading market to develop. We do not intend to apply to list the Warrants on any securities exchange or other trading market. Without a trading market, the liquidity of the Warrants will be extremely limited.
Right as a Stockholder
Except as otherwise provided in the Warrants or by virtue of the holder’s ownership of shares of Series B Preferred Stock, such holder of Warrants does not have the rights or privileges of a holder of Series B Preferred Stock, including any voting rights, until such holder exercises such holder’s Warrants.
Waivers and Amendments
No term of the Warrants may be amended or waived without the written consent of the holder of such warrant.
Pre-Funded Warrants
The following is a summary of all material terms and provisions of the Pre-Funded Warrants that are being ~~issued~~offered hereby, the form of which has been filed as an exhibit to the registration statement of which this prospectus is a part. Prospective investors should carefully review the terms and provisions of the form of Pre-Funded Warrant for a complete description of the terms and conditions of the Pre-Funded Warrants.
Pre-funded warrants provide any purchaser in this offering with the ability to purchase Pre-Funded Units (each Pre-Funded Unit consisting of one Pre-Funded Warrant to purchase one share of Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock) in lieu of Units that would otherwise result in the purchaser’s beneficial ownership exceeding 4.99% of our outstanding Common Stock (or, at the election of the purchaser, 9.99%). This is accomplished through purchasing Pre-Funded Warrants at a price equal to the purchase price for Units, less $0.01, which $0.01 is the exercise price for the Pre-Funded Warrants. Each Pre-Funded Warrant is exercisable into one share of Common Stock as offered hereunder. Thus, the purchaser is paying essentially the purchase price for a Unit at closing of the offering but is not deemed to beneficially own the shares of Common Stock included in the Units until the purchaser exercises the Pre-Funded Warrant. Once purchased, the purchase price of the Pre-Funded Warrants is not refundable. While the Pre-Funded Warrants permit waiver of provisions by us and the holder of the Pre-Funded Warrants, this would not affect the pre-funding as that is the purchase price of the instrument which is paid at the time of closing and becomes part of our proceeds received from this offering. In addition, the Pre-Funded Warrants are perpetual and do not have an expiration date.
Duration and Exercise Price
Each Pre-Funded Warrant will have an outstanding exercise price per share equal to $0.01. The Pre-Funded Warrants will be immediately exercisable and may be exercised at any time until the Pre-Funded Warrants are exercised in full. The exercise price and number of shares of Common Stock issuable upon exercise is subject to appropriate adjustment in the event of stock dividends, stock splits, reorganizations or similar events affecting Common Stock and the exercise price. The Pre-Funded Warrants will be issued separately from the accompanying Warrants included in the Pre-Funded Units, and may be transferred separately immediately thereafter.
Exercisability
The Pre-Funded Warrants will be exercisable, at the option of each holder, in whole or in part, by delivering to us a duly executed exercise notice accompanied by payment in full for the number of shares of Common Stock purchased upon such exercise (except in the case of a cashless exercise as discussed below). A holder (together with its affiliates) may not exercise any portion of the Pre-Funded Warrant to the extent that the holder would own more than 4.99% of the outstanding Common Stock immediately after exercise, except that upon at least 61 days’ prior notice from the holder to us, the holder may increase the amount of ownership of outstanding Common Stock after exercising the holder’s Pre-Funded Warrants up to 9.99% of the number of shares of Common Stock outstanding immediately after giving effect to the exercise, as such percentage ownership is determined in accordance with the terms of the Pre-Funded Warrants. Purchasers of Pre-Funded Units in this offering may also elect prior to the issuance of the Pre-Funded Warrants to have the initial exercise limitation set at 9.99% of our outstanding Common Stock.
Cashless Exercise
If, at the time a holder exercises its Pre-Funded Warrants, there is no effective registration statement registering, or the prospectus contained therein is not available for an issuance of the shares of Common Stock underlying the Pre-Funded Warrants to the holder, then in lieu of making the cash payment otherwise contemplated to be made to us upon such exercise in payment of the aggregate exercise price, the holder may elect instead to exercise its Pre-Funded Warrants on a cashless basis and receive upon such exercise (either in whole or in part) the net number of shares of Common Stock determined according to a formula set forth in the Pre-Funded Warrant.
Fundamental Transactions
In the event of any fundamental transaction, as described in the Pre-Funded Warrants and generally including any merger with or into another entity, sale of all or substantially all of our assets, tender offer or exchange offer, or reclassification of Common Stock, then upon any subsequent exercise of a Pre-Funded Warrant, the holder will have the right to receive as alternative consideration, for each share of Common Stock that would have been issuable upon such exercise immediately prior to the occurrence of such fundamental transaction, the number of shares of Common Stock of the successor or acquiring corporation or of our company, if it is the surviving corporation, and any additional consideration receivable upon or as a result of such transaction by a holder of the number of shares of Common Stock for which the Pre-Funded Warrant is exercisable immediately prior to such event.
Transferability
Subject to applicable laws, a Pre-Funded Warrant may be transferred at the option of the holder upon surrender of the Pre-Funded Warrant to us together with the appropriate instruments of transfer.
Fractional Shares
No fractional shares of Common Stock will be issued upon the exercise of the Pre-Funded Warrants. Rather, the number of shares of Common Stock to be issued will be rounded up to the nearest whole number.
Trading Market
There is no established trading market for the Pre-Funded Warrants on any securities exchange or nationally recognized trading system, and we do not expect an active trading market to develop. We do not intend to list the
Pre-Funded Warrants on any securities exchange or other trading market. Without a trading market, the liquidity of the Pre-Funded Warrants will be extremely limited.
Right as a Stockholder
Except as otherwise provided in the Pre-Funded Warrants or by virtue of the holder’s ownership of shares of Common Stock, such holder of Pre-Funded Warrants does not ~~make~~have the rights or privileges of a holder of Common Stock, including any ~~recommendation~~voting rights, until such holder exercises such holder’s Pre-Funded Warrants.
PRINCIPAL STOCKHOLDERS
The following table sets forth certain information regarding the beneficial ownership of Common Stock as of March 31, 2018 by (a) each person known by us to be the beneficial owner of more than 5% of the outstanding shares of Common Stock, (b) each of our named executive officers, (c) each of our directors, and (d) all of our executive officers and directors as a group.
The percentage of Common Stock outstanding is based on 19,426,163 shares of Common Stock outstanding on March 31, 2018. For purposes of the table below, and in accordance with the rules of the SEC, we deem shares of Common Stock subject to warrants and options that are currently exercisable or exercisable within sixty days of March 31, 2018 to be outstanding and to be beneficially owned by the person holding the warrants and options for the purpose of computing the percentage ownership of that person, but we do not treat them as outstanding for the purpose of computing the percentage ownership of any other person. Except as otherwise noted, each of the persons or entities in this table has sole voting and investing power with respect to ~~such Subscription Rights (including~~all of the shares of Common Stock beneficially owned by him, her or it, subject to community property laws, where applicable. Except as otherwise noted below, the street address of each beneficial owner is c/o Onconova Therapeutics, Inc., 375 Pheasant Run, Newtown, PA 18940.

Number of

Percentage of

Shares

Shares

Beneficially

Beneficially

Name and Address of Beneficial Owner

Owned

Owned

5% or greater stockholders:

The Michael and Jane Hoffman 2013

1,377,306

6.9
%
Descendants Trust(1)

712 Fifth Avenue, 51st Fl.

New York, NY 10019

Michael B. Hoffman(2)

1,407,419

7.1
%
(Includes The Michael and Jane

Hoffman 2013 Descendants Trust)

712 Fifth Avenue, 51st Fl

New York, NY 10019

Tyndall Capital Partners, L.P.(3)

1.387.646

12.4
%
405 Park Avenue, Suite 1104

New York, NY 10022

683 Capital Partners, LP, 683 Capital Management, LLC,

2,664,934

14.4
%
and Ari Zweiman(14)

3 Columbus Circle, Suite 2205

New York, NY 10019

Sabby Healthcare Master Fund, Ltd.,

Sabby Volatility Warrant Master Fund, Ltd., Sabby Management, LLC and Hal Mintz (15)

561,163

5.21
%

Sabby Healthcare Master Fund, Ltd.

c/o Ogier Fiduciary Services (Cayman) Limited

89 Nexus Way, Camana Bay

Grand Cayman KY1-9007

Cayman Islands

Sabby Volatility Warrant Master Fund, Ltd.

c/o Ogier Fiduciary Services (Cayman) Limited

89 Nexus Way, Camana Bay

Grand Cayman KY1-9007

Cayman Islands

Sabby Management, LLC

10 Mountainview Road, Suite 205

Upper Saddle River, New Jersey 07458

Hal Mintz

c/o Sabby Management, LLC

10 Mountainview Road, Suite 205

Upper Saddle River, New Jersey 07458

Armistice Capital, LLC, Armistice Capital Master Fund Ltd., and

1,607,500

9.0
%
Steven Boyd(16)

Armistice Capital, LLC

510 Madison Avenue, 22nd Floor

New York, New York 10022

Armistice Capital Master Fund Ltd.

c/o dms Corporate Services Ltd.

20 Genesis Close

P.O. Box 314

Grand Cayman KY1-1104

Cayman Islands

Steven Boyd

c/o Armistice Capital, LLC

510 Madison Avenue, 22nd Floor

New York, New York 10022

Other Directors, Director Nominees and Named Executive Officers:

Henry S. Bienen, Ph.D.(5)

27,438

*

Jerome E. Groopman, M.D.(6)

10,985

*

Ramesh Kumar, Ph.D.(7)

371,304

1.9
%
Manoj Maniar, Ph.D.(8)

61,280

*

James J. Marino(9)

158,969

*

Steven M. Fruchtman, M.D.(10)

72,911

*

Viren Mehta(11)

19,877

*

E. Premkumar Reddy, Ph.D.(4)

492,832

2.5
%
Jack E. Stover(12)

3,785

*

All current executive officers, directors and director nominees as a group (11 persons)(13)

2,675,159

13.1
%
*
Represents a beneficial ownership of less than one percent of our outstanding Common Stock.
(1) Includes 396,633 shares of Common Stock issuable upon the exercise of warrants that are currently exercisable or exercisable within sixty days of March 31, 2018.
(2) Includes (i) 1,377,306 shares of Common Stock beneficially owned by the Michael and Jane Hoffman 2013 Descendants Trust of which Mr. Hoffman is donor, (ii) 8,453 shares of Common Stock held by the Michael and Jane Hoffman 2013 Descendants Trust (Non-GST Exempt Trust) of which Mr. Hoffman is donor and (iii) 19,784 shares of Common Stock subject to outstanding options that are exercisable within 60 days of March 31, 2018. Mr. Hoffman has no voting or dispositive power with ~~respect~~regard to any of the shares held by the Michael and Jane Hoffman 2013 Descendants Trust and the Michael and Jane Hoffman 2013 Descendants Trust (Non-GST Exempt Trust). A.J.
Agarwal and Jane Hoffman, Mr. Hoffman’s spouse, as trustees, have voting and dispositive power with regard to the ~~exercise or expiration~~shares held by the Michael and Jane Hoffman 2013 Descendants Trust and the Michael and Jane Hoffman 2013 Descendants Trust (Non-GST Exempt Trust).
(3) Based solely on a Schedule 13G/A filed with the SEC on February 14, 2018. The Schedule 13G/A was filed on behalf of ~~such Subscription Rights)~~Tyndall Capital Partners, L.P. As of December 31, 2017, Tyndall Partners, L.P., ~~Units,~~a Delaware limited partnership held, 997,022 shares of ~~common stock~~Common Stock and warrants to purchase 390,624 shares of Common Stock. Tyndall Capital Partners, L.P. is the general partner of Tyndall Partners, L.P. and possesses the sole power to vote and the sole power to direct the disposition of all the shares and warrants. Jeffrey Management, LLC is the general partner of Tyndall Capital Partners, L.P. Jeffrey S. Hallis is the manager of Jeffrey Management, LLC.
(4) Includes 107,000 shares of Common Stock issuable upon the exercise of warrants and options that are currently exercisable or ~~Warrants.~~exercisable within sixty days of March 31, 2018.

(5) Includes 14,714 shares of Common Stock issuable upon the exercise of warrants and options that are currently exercisable or exercisable within sixty days of March 31, 2018.
(6) Includes 10,975 shares of Common Stock issuable upon the exercise of options that are currently exercisable or exercisable within sixty days of March 31, 2018.
(7) Includes (i) 37,510 shares of Common Stock held by the Ramesh Kumar 2012 Trust and (ii) 269,077 shares of Common Stock subject to outstanding warrants and options that are exercisable within 60 days of March 31, 2018. Dr. Kumar has voting and dispositive power with regard to the shares held by the Ramesh Kumar 2012 Trust.
(8) Includes 61,230 shares of Common Stock issuable upon the exercise of options that are currently exercisable or exercisable within sixty days of March 31, 2018.
(9) Includes 72,648 shares of Common Stock issuable upon the exercise of warrants and options that are currently exercisable or exercisable within sixty days of March 31, 2018.
(10) Includes 66,535 shares of Common Stock issuable upon the exercise of warrants and options that are currently exercisable or exercisable within sixty days of March 31, 2018.
(11) Includes (i) 2,844 shares of Common Stock held jointly with Dr. Mehta’s spouse, (ii) 806 shares of Common Stock held by Mehta Partners, LLC, (iii) 174 shares of Common Stock held by Mehta Partners, LLC FBO Jean Marie Kiss IRA, (iv) 830 shares of Common Stock held by Viram Foundation and (v) 5,375 shares of Common Stock subject to outstanding options that are exercisable within 60 days of March 31, 2018. Dr. Mehta, as managing member, has voting and dispositive power with regard to the shares held by Mehta Partners, LLC. Dr. Mehta, as trustee, has voting and dispositive power with regard to the shares held by Mehta Partners, LLC FBO Jean Marie Kiss IRA. Dr. Mehta, as trustee has voting and dispositive power with regard to the shares held by Viram Foundation.
(12) Includes 3,775 shares of Common Stock issuable upon the exercise of options that are currently exercisable or exercisable within sixty days of March 31, 2018.
(13) Includes 1,070,950 shares of Common Stock issuable upon the exercise of options that are currently exercisable or exercisable within sixty days of March 31, 2018.
(14) Based solely on a Form 4 (the “683 Capital Form 4”) filed with the SEC on February 12, 2018, except that the 14.4% was calculated using the 2,664,934 share reported in the 683 Capital Form 4 divided by 18,526,163 shares of Common Stock outstanding on March 31, 2018. The 683 Capital Form 4 was filed jointly by 683 Capital Management, LLC (“683 Management”), 683 Capital Partners, LP (“683 Partners”) and Ari Zweiman (collectively, the “683 Capital Reporting Persons”). According to the 683 Capital Form 4, each of the 683 Capital Reporting Persons may be deemed to be a member of a Section 13(d) group that collectively owns more than 10% of the Issuer’s outstanding shares of Common Stock. Each of the 683 Capital Reporting Persons disclaims beneficial ownership of the securities reported herein except to the extent of his or its pecuniary interest therein. In ~~connection~~addition, according to the 683 Capital Form 4, as of February 8, 2018, 2,664,934 shares of Common Stock were held directly by 683 Partners.
683 Management is the investment manager of 683 Partners. Ari Zweiman is the Managing Member of 683 Management. As a result, each of 683 Management and Ari Zweiman may be deemed to beneficially own the securities held by 683 Partners.
(15) Based solely on a Schedule 13G/A filed with the SEC on January 9, 2018. The Schedule 13G/A was filed by Sabby Healthcare Master Fund, Ltd., Sabby Volatility Master Fund, Ltd. and Sabby Management, LLC. According to the Schedule 13G/A, as of December 31, 2017: (i) Sabby Healthcare Master Fund, Ltd. and Sabby Volatility Master Fund, Ltd. beneficially own 440,300 and 120,863 shares of Common Stock, respectively, representing approximately 4.09% and 1.12% of Common Stock, respectively, and (ii) Sabby Management, LLC and Hal Mintz each beneficially own 561,163 shares of Common Stock, representing approximately 5.21% of Common Stock. Sabby Management, LLC and Hal Mintz do not directly own any shares of Common Stock, but each indirectly owns 561,163 shares of Common Stock. Sabby Management, LLC, a Delaware limited liability company, indirectly owns 561,163 shares of Common Stock because it serves as the investment manager of Sabby Healthcare Master Fund, Ltd. and Sabby Volatility Warrant Master Fund, Ltd., Cayman Islands companies. Mr. Mintz indirectly owns 561,163 shares of Common Stock in his capacity as manager of Sabby Management, LLC.
(16) Based solely on a Schedule 13G filed with the SEC on February 20, 2018. The Schedule 13G was filed by Armistice Capital, LLC, Armistice Capital Master Fund Ltd., and Steven Boyd. According to the Schedule 13G, Armistice Capital, LLC, Armistice Capital Master Fund Ltd. and Steven Boyd each had (i) sole power to vote or direct the vote of 0 share, (ii) shares power to vote or direct the vote of 1,607,500 shares of Common Stock, or 9.0% of outstanding shares of Common Stock, (iii) sole power to dispose or to direct the disposition of 0 share and (iv) shared power to dispose or direct the disposition of 1,607,500 shares of Common Stock, or 9.0% of outstanding shares of Common Stock.
UNDERWRITING
We have entered into an underwriting agreement dated                    , 2018, with H.C. Wainwright & Co., LLC as the sole book-running manager of this ~~Rights Offering,~~offering. Subject to the terms and conditions of the underwriting agreement, we have agreed to ~~pay~~sell to the ~~dealer-manager~~underwriter and the underwriter has agreed to purchase from us, at the public offering price less the underwriting discounts and commissions set forth on the cover page of this prospectus. The public offering price shown on the cover page of this prospectus was determined by negotiation between us and the underwriter at the time of pricing, and may be at a ~~cash fee equal~~discount to ~~(a) 3.5%~~the current market price.
A copy of the ~~dollar amount~~underwriting agreement has been filed as an exhibit to the registration statement of which this prospectus is a part. The shares we are offering are being offered by the underwriter subject to certain conditions specified in the underwriting agreement.
We have been advised by the underwriter that it proposes to offer the shares directly to the public at the public offering price set forth on the cover page of this prospectus. Any shares sold by the underwriter to securities dealers will be sold at the public offering price less a selling concession not in excess of $            per share.
The underwriting agreement provides that the underwriter’s obligation to purchase the securities we are offering is subject to conditions contained in the underwriting agreement. The underwriter is obligated to purchase and pay for all of the ~~Units sold to any holders of Subscription Rights who were beneficial owners of shares~~securities offered by this prospectus.
No action has been taken by us or the underwriter that would permit a public offering of our ~~common stock prior to July 30, 2013, and (b) 7.0%~~securities in any jurisdiction where action for that purpose is required. None of the ~~dollar amount of the Units~~securities included in this offering may be offered or sold, todirectly or indirectly, nor may this prospectus or any other holders of Subscription Rights. We have also provided to the dealer-manager a non-accountable expense allowance of $100,000 for expenses incurred in connection with this Rights Offering. We advanced $30,000 of this $100,000 allowance to Maxim Group LLC upon its engagement as a dealer-manager. If its engagement is terminated, Maxim

~~Table of Contents~~
~~Group LLC will promptly reimburse to us any portion of the advance not used for actual out-of-pocket expenses~~offering material or advertisements in connection with the ~~Rights Offering.~~offer and sales of any of the securities be distributed or published in any jurisdiction, except under circumstances that will result in compliance with the applicable rules and regulations of that jurisdiction. Persons who receive this prospectus are advised to inform themselves about and to observe any restrictions relating to this offering of the securities and the distribution of this prospectus. This prospectus is neither an offer to sell nor a solicitation of any offer to buy the securities in any jurisdiction where that would not be permitted or legal.
The underwriter has advised us that it does not intend to confirm sales to any accounts over which it exercises discretionary authority.
Underwriting Discounts, Commissions and Expenses
We have agreed to pay an underwriter discount equal to 7% of the aggregate gross proceeds raised in this offering.
The following table shows the public offering price, underwriting discounts and commissions and proceeds, before expenses to us. These amounts are shown assuming both no exercise and full exercise of the underwriter’s option to purchase additional securities.
Total
Per Unit
Per
Pre-Funded
Unit
Without
Option
Exercise
With
Full Option
Exercise
Public offering price
Underwriting discounts and commissions
Proceeds, before expenses, to us
We estimate the total expenses payable by us for this offering to be approximately $1.6 million, which amount includes (i) an assumed underwriting discounts and commissions of $1.1 million ($1.2 million if the underwriter’s option to purchase additional securities is exercised in full) based upon the assumed public offering price per Unit of $0.56 (the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018), (ii) an assumed management fee in the amount of $150,000 which represents 1.0% of the assumed aggregate offering price,
(iii) $50,000 non-accountable expense allowance payable to the underwriter, (iv) reimbursement of the accountable expenses of the underwriter equal to $110,000 (none of which has been paid in advance), including the legal fees of the underwriter being paid by us, and (v) other estimated expenses of approximately $289,000 which include legal, accounting, printing costs and various fees associated with the registration and listing of our shares.
Underwriter Warrants
We have agreed to issue to the underwriter warrants to purchase a number of shares of Common Stock equal to 5.0% of the aggregate number of the Units and Pre-Funded Units sold in this offering, or warrants to purchase up to 1,339,286 of our Common Stock (excluding the underwriter’s option to purchase additional securities). The underwriter warrants will have substantially the same terms as the terms of the Warrants offered pursuant to this prospectus, except that the exercise price per share equal to 125% of the public offering price for the shares sold in this offering, or $0.70 (based on an assumed public offering price of $0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018), the warrants will be exercisable into shares of Common Stock and the exercise period of the underwriter warrants shall be subject to the limitation on exercise set forth in FINRA Rule 5110(f)(2)(G)(i). Pursuant to FINRA Rule 5110(g), the underwriter warrants and any shares issued upon exercise of the underwriter warrants shall not be sold, transferred, assigned, pledged, or hypothecated, or be the subject of any hedging, short sale, derivative, put or call transaction that would result in the effective economic disposition of the securities by any person for a period of 180 days immediately following the date of effectiveness or commencement of sales of this offering, except the transfer of any security: (i) by operation of law or by reason of our reorganization; (ii) to any FINRA member firm participating in the offering and the officers or partners thereof, if all securities so transferred remain subject to the lock-up restriction set forth above for the remainder of the time period; (iii) if the aggregate amount of our securities held by the underwriter or related persons do not exceed 1% of the securities being offered; (iv) that is beneficially owned on a pro rata basis by all equity owners of an investment fund, provided that no participating member manages or otherwise directs investments by the fund and the participating members in the aggregate do not own more than 10% of the equity in the fund; or (v) the exercise or conversion of any security, if all securities remain subject to the lock-up restriction set forth above for the remainder of the time period.
Right of First Refusal
We have also granted the underwriter, for a period of 10 months from the closing date of this offering, a right of first refusal to act as sole book-running manager for each and every future public or private equity or debt offering by us or any of our successors or subsidiaries. We have also agreed to a tail fee equal to the cash and warrant compensation in this offering if any investor to which the underwriter introduced us with respect to this offering during the term of its engagement provides us with further capital in a public or private offering or capital raising transaction, with certain exceptions, during the 6-month period following termination of our engagement of the underwriter.
Option to Purchase Additional Securities
We have granted the underwriter the option to purchase up to 4,017,857 additional shares of Common Stock at a purchase price of $      per share and/or Warrants to purchase up to an aggregate of 100,446 shares of Series B Preferred Stock at a purchase price of $0.01 per Warrant with an exercise price of $            per 0.025 share of Series B Preferred Stock, less the underwriting discounts and commissions. The underwriter may exercise its option at any time and from time to time within 30 days from the date of this prospectus. If any additional securities are purchased pursuant to the option, the underwriter will offer these securities on the same terms as those on which the other securities are being offered hereby.
Listing
Common Stock is currently traded on the Nasdaq Capital Market under the symbol “ONTX.” On April 20, 2018, the last reported sale price of Common Stock was $0.56 per share. We do not intend to apply for listing of the Pre-Funded Warrants, the Warrants or the Series B Preferred Stock on any securities exchange or other nationally recognized trading system. There is no established public trading market for the Pre-Funded Warrants, the Warrants or the Series B Preferred Stock, and we do not expect a market to develop.
Lock-up Agreements
Our officers and directors and certain of our stockholders have agreed with the underwriter to be subject to a lock-up period of 90 days following the date of this prospectus. This means that, during the applicable lock-up period, such persons may not offer for sale, contract to sell, sell, distribute, grant any option, right or warrant to purchase, pledge, hypothecate or otherwise dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into, or exercisable or exchangeable for, shares of Common Stock. Certain limited transfers are permitted during the lock-up period if the transferee agrees to these lock-up restrictions. We have also agreed in the underwriting agreement, subject to certain exceptions, to similar lock-up restrictions on the issuance and sale of our securities for 90 days following the closing of this offering. The underwriter may, in its sole discretion and without notice, waive the terms of any of these lock-up agreements.
Insider Participation
One or more of our directors have indicated interests in purchasing approximately 10% of the Units to be sold in this offering at the public offering price and on the same terms as the other purchasers in this offering. However, because indications of interest are not binding agreements or commitments to purchase, the underwriter could determine to sell more, fewer or no Units to our director(s) in this offering, or our director(s) could determine to purchase more, fewer or no Units in this offering.
Company Lock-Up Waiver Agreement
In connection with our underwritten public offering of securities closed on February 12, 2018 (the “February 2018 Offering”), we agreed to certain restrictions (the “Company Lock-Up”) set forth in Section 5(j) of the Underwriting Agreement dated as of February 8, 2018 (the “February 2018 Underwriting Agreement”) between us and H.C. Wainwright & Co., LLC, as the underwriter for the February 2018 Offering. Under the Company Lock-Up, during a period of one hundred and thirty-five (135) days from February 8, 2018, we will not, without the prior written consent of H.C. Wainwright & Co., LLC, (i) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend or otherwise transfer or dispose of, directly or indirectly, or announce the issuance or proposed issuance of, any Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock or (ii) enter into any swap or other agreement that transfers, in whole or in part, any of the economic consequences of ownership of Common Stock, whether any such transaction described in clause (i) or (ii) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise, subject to certain exceptions.
In connection with this offering, on April 16, 2018, we entered into a Lock-Up Waiver Agreement (the “Lock-Up Waiver Agreement”) with H.C. Wainwright & Co., LLC and certain holders of the Company’s preferred stock warrants issued in the February 2018 Offering (the “Preferred Stock Warrants”), pursuant to which (i) H.C. Wainwright & Co., LLC waived the Company Lock-Up solely with respect to this offering, and (ii) we agree to reduce the exercise price of the Preferred Stock Warrants such that the exercise price of the Preferred Stock Warrants shall be equal to 105% of the public offering price of Common Stock sold in this offering (but only to the extent that such public offering price is lower than the current exercise price of the Preferred Stock Warrants) and that such repricing shall be effective concurrently with the closing of this offering.
Stabilization, Short Positions and Penalty Bids
The underwriter may engage in syndicate covering transactions, stabilizing transactions and penalty bids or purchases for the purpose of pegging, fixing or maintaining the price of Common Stock:
· Syndicate covering transactions involve purchases of securities in the open market after the distribution has been completed in order to cover syndicate short positions. Such a naked short position would be closed out by buying securities in the open market. A naked short position is more likely to be created if the underwriter is concerned that there could be downward pressure on the price of the securities in the open market after pricing that could adversely affect investors who purchase in the offering.
· Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specific maximum.
· Penalty bids permit the underwriter to reclaim a selling concession from a syndicate member when the securities originally sold by the syndicate member are purchased in a stabilizing or syndicate covering transaction to cover syndicate short positions.
These syndicate covering transactions, stabilizing transactions and penalty bids may have the effect of raising or maintaining the market prices of our securities or preventing or retarding a decline in the market prices of our securities. As a result, the price of Common Stock may be higher than the price that might otherwise exist in the open market. Neither we nor the underwriter make any representation or prediction as to the effect that the transactions described above may have on the price of Common Stock. These transactions may be effected on the Nasdaq Capital Market, in the over-the-counter market or on any other trading market and, if commenced, may be discontinued at any time.
In connection with this offering, the underwriter also may engage in passive market making transactions in Common Stock in accordance with Regulation M during a period before the commencement of offers or sales of shares of Common Stock in this offering and extending through the completion of the distribution. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for that security. However, if all independent bids are lowered below the passive market maker’s bid that bid must then be lowered when specific purchase limits are exceeded. Passive market making may stabilize the market price of the securities at a level above that which might otherwise prevail in the open market and, if commenced, may be discontinued at any time.
Neither we nor the underwriter make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the prices of our securities. In addition, neither we nor the underwriter make any representation that the underwriter will engage in these transactions or that any transactions, once commenced, will not be discontinued without notice.
Indemnification
We have agreed to indemnify the ~~dealer-manager and its respective affiliates~~underwriter against certain liabilities, including certain liabilities arising under the Securities ~~Act.~~ Act, or to contribute to payments that the underwriter may be required to make for these liabilities.
Other Relationships
The ~~dealer-manager's participation~~underwriter and its respective affiliates have engaged in, ~~this offering is subject to customary conditions contained~~and may in the ~~dealer-manager agreement, including the receipt by the dealer-manager of an opinion of our counsel. The dealer-manager~~future engage in, investment banking and ~~its affiliates may provide to us from time to time in the future~~other commercial dealings in the ordinary course of ~~their~~ business ~~certain financial advisory, investment banking~~with us or our affiliates. The underwriter has received, or may in the future receive, customary fees and ~~other services~~commissions for these transactions. The underwriter of this offering also acted as underwriter in our offering consummated in February 2018, for which ~~they will be entitled to receive fees.~~it received compensation.
EXPERTS
Maxim Group LLC is a broker-dealer and member of the Financial Industry Regulatory Authority, Inc. The principal business address of Maxim Group LLC is 405 Lexington Avenue, New York, New York 10174.

~~EXPERTS~~
The consolidated financial statements of Onconova Therapeutics, Inc. at December 31, 2017 and 2016, and for the years then ended, appearing in ~~Onconova Therapeutics, Inc.'s~~our Annual Report (Form 10-K) for the year ended December 31, ~~2015~~2017 have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon (which contains an explanatory paragraph describing conditions that raise substantial doubt about the ~~Company's~~Company’s ability to continue as a going concern as described in Note 1 to the consolidated financial statements), included therein and incorporated herein by reference. Such ~~consolidated~~ financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

LEGAL MATTERS

The validity of the ~~shares of common stock~~securities being offered ~~hereby and certain other legal matters~~by this prospectus will be passed upon ~~for us~~ by ~~Pepper Hamilton~~Morgan, Lewis & Bockius LLP, Philadelphia, Pennsylvania. The underwriter is being represented in connection with this offering by Lowenstein Sandler LLP, New York, New York.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy and information statements and other information with the ~~Securities and Exchange Commission, or "SEC." You may read and copy any documents we file with~~SEC. Our SEC filings, including the SEC at the SEC's Public Reference Room at 100 F Street, NE, Washington, D.C. 20549. You may obtain information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC-0330. In addition, our filings with the SECregistration statement, are available to the public ~~through~~from the ~~SEC's Internet site~~SEC’s website at ~~http://~~www.sec.gov. ~~Information about us is~~To receive copies of public records not posted to the SEC’s website at prescribed rates, you may complete an online form at www.sec.gov, send a fax to (202) 772-9337 or submit a written request to the SEC, Office of FOIA/PA Operations, 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information.
We also make available free of charge on our website, ~~at http://www.onconova.com. This URL~~www.onconova.com, all materials that we file electronically with the SEC, including our annual reports on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K, Section 16 reports and amendments to those reports as soon as reasonably practicable after such materials are electronically filed with, or furnished to, the ~~SEC's URL above are intended to be inactive textual references only. The information~~SEC. Information contained on ~~the SEC's~~our website ~~and our~~or any other website is not incorporated by reference into this prospectus and does not constitute a part of ~~and is not incorporated into,~~ this prospectus.
We have filed a registration statement covering our shares of common stock subject to this offering, of which this prospectus forms a part. This prospectus, however, does not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information concerning us and the securities we may offer and sell, you should read the entire registration statement and the exhibits to the registration statement. The registration statement has been filed electronically and may be obtained in any manner listed above. Any statements contained in this prospectus concerning the provisions of any document are not necessarily complete, and, in each instance, reference is made to the copy of such document filed as an exhibit to the registration statement or otherwise filed with the SEC. Each such statement is qualified in its entirety by such reference.

~~Table of Contents~~

INCORPORATION BY REFERENCE

The SEC allows us to ~~"incorporate~~“incorporate by ~~reference"~~reference” the information we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this ~~prospectus.~~prospectus, and information that we file later with the SEC will automatically update and supersede this information. We incorporate by reference the documents listed below:
•
·Our Annual Report on Form 10-K for the fiscal year ended December 31, ~~2015, which~~2017 that we filed with the SEC on March ~~28, 2016;~~16, 2018;
·

•
~~Our Definitive Proxy Statement on Schedule 14A~~                  Information required by Part III of Form 10-K contained in our definitive proxy statement for our ~~2016 Annual~~2017 annual meeting of stockholders;
·      Our definitive proxy statement for our 2018 Special Meeting of Stockholders ~~held May 18, 2016, which we~~ filed ~~with the SEC~~ on ~~April 13, 2016;~~February 28, 2018;
·

•
Our ~~Quarterly Report on Form 10-Q for the fiscal period ended March 31, 2016, which we filed with the SEC on May 11, 2016;~~

•
~~Our current reports~~Current Reports on Form 8-K filed with the SEC on January 17, 2018, January 30, 2018, February 6, ~~2016,~~2018, February ~~4, 2016, February 17, 2016,~~8, 2018, March ~~9, 2016, May~~8, 2018, March 22, 2018, April 19, 2018 and April 23, ~~2016 and May 31, 2016;~~2018;
·

•
The description of ~~our common stock~~Common Stock contained in our registration statement on Form 8-A filed on July 23, 2013 (Registration no. 001-36020) with the SEC, including any amendment or report filed for the purpose of updating such description; and
·

•
All documents filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, or the Exchange Act, after the date of the initial filing of the registration statement of which this prospectus is a part and prior to the effectiveness of such registration statement; and

•
All documents filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date of this prospectus and before we ~~stop~~terminate the offering ~~the securities~~ under this prospectus.
We will provide without charge to each person, including any beneficial owner, to whom this prospectus is delivered, upon his or her written or oral request, a copy of any or all documents referred to above which have been or may be incorporated by reference into this prospectus but not delivered with this prospectus excluding exhibits to those documents unless they are specifically incorporated by reference into those documents. You can request those documents from us, at no cost, by writing or telephoning us at: Onconova Therapeutics, Inc., 375 Pheasant Run, Newtown, Pennsylvania, 18940, (267) ~~759-3036,~~759-3680, Attention: ~~Benjamin Hoffman.~~Suzanne Hutchinson.

The most recent information that we file with the SEC automatically updates and supersedes older information. The information contained in any such filing will be deemed to be a part of this prospectus, commencing on the date on which the filing is made.

Information furnished under Items 2.02 or 7.01 (or corresponding information furnished under Item 9.01 or included as an exhibit) in any past or future Current Report on Form 8-K that we file with the SEC, unless otherwise specified in such report, is not incorporated by reference in this prospectus.
Onconova Therapeutics, Inc.
Up to 26,785,714 Units (each Unit contains one Share of Common Stock and one Warrant to purchase 0.025 share of Series B Convertible Preferred Stock
Up to 26,785,714 Pre-Funded Units (each Pre-Funded Unit contains one Pre-Funded Warrant to purchase one Share of Common Stock and one Warrant to purchase 0.025 share of Series B Convertible Preferred Stock)
(Up to 669,643 shares of Series B Convertible Preferred Stock Underlying the Warrants) and
(Up to 26,785,714 Shares of Common Stock Underlying the Pre-Funded Warrants)
PROSPECTUS
Sole Book-Running Manager
H.C. Wainwright & Co.
                       , 2018

Delaware		2834		22-3627252
~~Delaware~~ (State or other jurisdiction of incorporation or organization)		~~2834~~ (Primary Standard Industrial Classification Code Number)		~~22-3627252~~ (I.R.S. Employer Identification No.)

Copy to: ~~Jeffery P. Libson~~ ~~Donald R. Readlinger~~ ~~Pepper Hamilton LLP~~ ~~301 Carnegie Center, Suite 400~~ ~~Princeton, NJ 08540-6227~~ ~~609-951-4164~~
Joanne R. Soslow		Steven M. Skolnick
Morgan, Lewis & Bockius LLP		Michael J. Lerner
1701 Market Street		Lowenstein Sandler LLP
Philadelphia, PA 19103		1251 Avenue of the Americas
(215) 963-5000		New York, New York 10022
		(212) 262-6700

Large accelerated filer	o	Accelerated filer			o
~~Large Accelerated~~ Non-accelerated filer	o		~~Accelerated filer~~ o	~~Non-accelerated filer~~ o ~~(Do~~ (Do not check if a smaller reporting company)		Smaller reporting companyý	x
		Emerging growth company	x

Title of securities to be registered	Proposed maximum aggregate offering price(1)	Amount of registration fee




Units, each consisting of share of common stock, par value $0.01 per share ("Common Stock") and warrant ("Warrant") to purchase one share of Common Stock ("Units")	$15,000,000	$1,511

Non-transferable Rights to purchase Units(2)	—	—

Common Stock, par value $0.01 per share included as part of the Units(3)	Included with Units above	—

Warrants included as part of the Units(3)	Included with Units above	—

Common Stock issuable upon exercise of the Warrants included in the Units(3)(4)	$7,500,000	755

Total	$22,500,000	$2,266


Title of Each Class of Securities To Be Registered	Proposed Maximum Aggregate Offering Price(1)(2)		Amount of Registration Fee(2)
Units, each Unit consisting of one share of common stock, par value $0.01 per share (“Common Stock”), and one warrant (“Warrant”) to purchase 0.025 share of Series B Convertible Preferred Stock, par value $0.01 per share (“Series B Preferred Stock”)(3)	$	17,250,000	$	2,147.63
(i) Common Stock included in the Units(4)	Included with the Units above		—
(ii) Warrants included in the Units(4)	Included with the Units above		—
Pre-Funded Units in lieu of Units, each Pre-Funded Unit consisting of one pre-funded warrant (“Pre-Funded Warrant”) to purchase one share of Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock(3)	Included with the Units above		—
(i) Pre-Funded Warrants included in the Pre-Funded Units(4)	Included with the Units above		—
(ii) Warrants included in the Pre-Funded Units(4)	Included with the Units above		—
Shares of Series B Preferred Stock underlying Warrants included in the Units and Pre-Funded Units	$	17,250,000	$	2,147.63
Shares of Common Stock underlying Pre-Funded Warrants included in the Pre-Funded Units(3)	Included with the Units above			—
Underwriter’s Warrants to purchase Common Stock(5)	—			—
Shares of Common Stock underlying Underwriter’s Warrants(6)	$	1,078,125	$	134.23
Total	$	35,578,125	$	4,429.49	(7)

	~~Per Unit~~	~~Total(2)~~
	Per Unit		Per Pre-Funded Unit	Total
Public offering price


~~Subscription price~~	$	$

~~Dealer-Manager fees and expenses(1)~~	$	$

~~Proceeds to us, before expenses~~	$		$

Underwriting discounts and commissions (1)	$		$	$

Proceeds, before expenses, to us (2)	$		$	$

	Overall Evaluable (N=33)		No prior HMA (N-20)		Prior HMA (N=13)
Complete remission (CR)	8(24	)%	7(35	)%	1(8	)%
Marrow CR + hematologic improvement	10(30	)%	6(30	)%	4(31	)%
Marrow CR alone	6(18	)%	3(15	)%	3(23	)%
Hematologic improvement alone	1(3	)%	1(5	)%	0
Stable disease	8(24	)%	3(15	)%	5(38	)%
Overall IWG response	25(76	)%	17(85	)%	8(62	)%
Clinical benefit response	19(58	)%	14(70	)%	5(38	)%

All Patients on Combination Part 1 (Rigosertib 840 mg total & Azacitidine)		42
Patients with hematuria		20 (48%)
Patients with grade 1 or 2 hematuria		17 (40%)
Patients with grade >3 hematuria		5 (12%)

All Patients on Combination Part 2 (Rigosertib 1120 mg total) & Azacitidine) with risk-mitigation strategies		37
Patients with hematuria		4 (11%)
Patients with grade 1 or 2 hematuria		4 (11%)
Patients with grade >3 hematuria		0 (0%)

	For the Years ended December 31,

	For the Years ended December 31,						For the three months ended March 31, 2016
	2015			2014			For the three months ended March 31, 2016
Revenue	$	11,456,000		$	800,000		$	1,474,000
Operating expenses:
General and administrative		9,533,000			15,119,000			3,172,000
Research and development		25,895,000			49,425,000			5,822,000

Total operating expenses		35,428,000			64,544,000			8,994,000

Loss from operations		(23,972,000	)		(63,744,000	)		(7,520,000	)
Change in fair value of warrant liability		—			20,000			271,000
Other income (expense), net		(35,000	)		(52,000	)		9,000

Net loss before income taxes		(24,007,000	)		(63,776,000	)		(7,240,000	)
Income taxes		16,000			19,000			—

Net loss		(24,023,000	)		(63,795,000	)		(7,240,000	)
Net loss attributable to non-controlling interest		44,000			113,000			—

Net loss attributable to Onconova Therapeutics, Inc		(23,979,000	)		(63,682,000	)	$	(7,240,000	)



Net loss per share of common stock, basic and diluted	$	(10.54	)	$	(29.41	)	$	(2.65	)



Basic and diluted weighted average shares outstanding		2,273,976			2,165,354			2,731,590

	December 31, 2015			March 31, 2016

Assets
Current assets:
Cash and cash equivalents	$	19,799,000		$	16,835,000
Receivables		1,504,000			1,368,000
Prepaid expenses and other current assets		1,832,000			1,153,000
Restricted cash		50,000			50,000

Total current assets		23,185,000			19,406,000
Property and equipment, net		248,000			224,000
Other non-current assets		12,000			12,000

Total assets	$	23,445,000		$	19,642,000



Liabilities and stockholders' equity
Current liabilities:
Accounts payable	$	3,421,000		$	3,296,000
Accrued expenses and other current liabilities		3,729,000			3,891,000
Deferred revenue		455,000			455,000

Total current liabilities		7,605,000			7,642,000
Warrant liability		—			295,000
Deferred revenue, non-current		5,000,000			4,886,000

Total liabilities		12,605,000			12,823,000

Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.01 par value, 5,000,000 authorized at March 31, 2016 and December 31, 2015, none issued and outstanding at March 31, 2016 and December 31, 2015		—			—
Common stock, $0.01 par value, 25,000,000 authorized at March 31, 2016 and December 31, 2015, 2,740,103 and 2,546,419 shares issued		25,000			27,000
Additional paid-in capital		328,564,000			331,775,000
Accumulated other comprehensive loss		(22,000	)		(16,000	)
Accumulated deficit		(318,557,000	)		(325,797,000	)

Total Onconova Therapeutics, Inc. stockholders' equity		10,010,000			5,989,000
Non-controlling interest		830,000			830,000

Total stockholders' equity		10,840,000			6,819,000

Total liabilities and stockholders' equity	$	23,445,000		$	19,642,000

Units offered by us in this offering	Up to 26,785,714 Units, each consisting of one share of Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock.
~~Securities to be offered~~
Pre-Funded Units offered by us in this offering	We are ~~distributing~~also offering to ~~you, at no charge, one non-transferable Subscription Right~~each purchaser whose purchase of Units in this offering would otherwise result in the purchaser, together with its affiliates and certain related parties, beneficially owning more than 4.99% of our outstanding Common Stock immediately following the consummation of this offering, the opportunity to purchase, if the purchaser so chooses, Pre-Funded Units (each Pre-Funded Unit consisting of one Unit for every share of our common stock that you owned on the Record Date, either as a holder of record or, in the case of shares held of record by brokers, banks, or other nominees, on your behalf, as a beneficial owner of such shares. Each Unit consists of share of common stock andPre-Funded Warrant ~~representing the right~~ to purchase one share of ~~common stock~~Common Stock and one Warrant to purchase 0.025 share of Series B Preferred Stock) in lieu of Units that would otherwise result in the purchaser’s beneficial ownership exceeding 4.99% of our outstanding Common Stock (or at the election of the purchaser, 9.99%). Each Pre-Funded Warrant contained in a Pre-Funded Unit will be exercisable for one share of Common Stock. The purchase price of each Pre-Funded Unit will equal the price per Unit being sold to the public in this offering minus $0.01, and the exercise price of each Pre-Funded Warrant included in the Pre-Funded Unit will be $0.01 per share of Common Stock. This offering also relates to the shares of Common Stock issuable upon exercise of any Pre-Funded Warrants contained in the Pre-Funded Units sold in this offering. For each Pre-Funded Unit we sell, the number of Units we are offering will be decreased on a one-for-one basis. Because we will issue a Warrant as part of each Unit or Pre-Funded Unit, the number of Warrants sold in this offering will not change as a result of a change in the mix of the Units and Pre-Funded Units sold.

Warrants offered by us in this offering	Warrants to purchase an aggregate of up to 669,643 shares of Series B Preferred Stock. Each Unit and each Pre-Funded Unit includes a Warrant to purchase 0.025 share of Series B Preferred Stock. Each Warrant contained in a Unit or Pre-Funded Unit has an exercise price of $ per ~~share.~~
	~~Holders of our currently outstanding participating warrants will also be entitled, pursuant to the terms of such warrants, to receive one Subscription Right to purchase one Unit for every~~0.025 share of ~~common stock~~Series B Preferred Stock, will be immediately separable from the Common Stock or Pre-Funded Warrant, as ~~if each~~the case may be, will be exercisable immediately and will expire on the 18-month anniversary of ~~such participating warrants had been exercised immediately prior to~~ the ~~record date for the Rights Offering.~~Charter Amendment Date.
~~Size~~
Series B Preferred Stock	This prospectus also relates to the offering of ~~offering~~	~~Units,~~the shares of Series B Preferred Stock issuable upon the exercise of the Warrants. The Series B Preferred Stock is not convertible until the next business day after the Charter Amendment Date, starting at which time each ~~consisting of~~0.025 share of ~~common stock and Warrant representing~~ the ~~right to purchase~~Series B Preferred Stock is convertible into one share of ~~common stock~~Common Stock. Notwithstanding the foregoing, we shall not effect any conversion of the Series B Preferred Stock, to the extent that, after giving effect to an attempted conversion, the holder of shares of Series B Preferred Stock (together with such holder’s affiliates, and any persons acting as a group together with such holder or any of such holder’s affiliates) would beneficially own a number of shares of Common Stock in excess of 4.99% of the shares of Common Stock then outstanding after giving effect to such exercise. In the event our stockholders do not approve the Charter Amendment, the Series B Preferred Stock will not be convertible into Common Stock and the value of the Warrants and the Series B Preferred Stock may be negatively affected. For additional information, see the subsection entitled “Description of Securities We Are Offering —Series B Convertible Preferred Stock” in this prospectus.

Insider Participation	One or more of our directors have indicated interests in purchasing approximately 10% of the Units to be sold in this offering at the public offering price and on the same terms as the other purchasers in this offering. However, because indications of interest are not binding agreements or commitments to purchase, the underwriter could determine to sell more, fewer or no Units to our director(s) in this offering, or our director(s) could determine to purchase more, fewer or no Units in this offering.

Offering Price	$ per Unit $ per Pre-Funded Unit

Option to purchase additional securities	The underwriter has the option to purchase up to 4,017,857 additional shares of Common Stock at a purchase price of $ per share and/or Warrants to purchase up to an aggregate of 100,446 shares of Series B Preferred Stock at a purchase price of $0.01 per Warrant with an exercise price of $ per ~~share.~~
~~Subscription Price~~	~~$ per Unit.~~
~~Warrants~~	~~Each Warrant entitles the holder to purchase one~~0.025 share of ~~common stock~~Series B Preferred Stock, less the underwriting discounts and commissions. The underwriter may exercise its option at ~~an exercise price of $ per share~~any time and from time to time within 30 days from the date of issuance through its expiration on . The Warrants will be exercisable by paying the exercise price in cash, or, solely during any period when a registration statement for the exercise of the Warrants is not in effect, exercisable on a cashless basis. We intend to apply to list the Warrants on NASDAQ under the symbol "ONTXW," although there is no assurance that a sufficient number of Subscription Rights will be exercised so that the Warrants will meet the minimum listing criteria to be accepted for listing on NASDAQ. We may redeem the Warrants for $ per Warrant if our common stock closes above $ per share for 10 consecutive trading days.this prospectus.
~~Record Date~~	~~5:00 PM Eastern Time, , 2016.~~
~~Subscription Right~~Common stock to be outstanding after this offering	~~Each Subscription Right consists~~46,211,877 shares of ~~a Basic Subscription Right and an Over-Subscription Privilege.~~Common Stock (assuming no sale of any Pre-Funded Units), or 50,229,734 shares of Common Stock if the underwriter exercises its option to purchase additional Units in full (assuming no sale of any

		Pre-Funded Units).
~~Basic Subscription Rights~~		~~Basic Subscription Right will entitle you to purchase one Unit at the Subscription Price.~~


~~Over-Subscription Privilege~~		If you exercise your Basic Subscription Rights in full, you may also choose to purchase a portion of any Units that are not purchased by our other shareholders or holders of participating warrants through the exercise of their Basic Subscription Rights. You may subscribe for additional Units pursuant to this Over-Subscription Privilege, subject to proration described elsewhere.
~~Expiration date~~Use of proceeds		~~The Subscription Rights will expire at 5:00 PM Eastern Time, on , 2016.~~ We ~~reserve~~estimate that the ~~right~~net proceeds to ~~extend the expiration date in our sole discretion.~~
~~Procedure for exercising Subscription Rights~~		~~To exercise your Subscription Rights, you must take the following steps:~~
		If you are a record holder of our common stock or a holder of participating warrants, you must deliver payment and a properly completed Subscription Rights Statement to the Subscription Agent to be received before 5:00 PM Eastern Time, on , 2016. You may deliver the documents and payments by first class mail or courier service. If you use first class mail forus from this ~~purpose, we recommend using registered mail, properly insured, with return receipt requested.~~
		If you are a beneficial owner of shares that are registered in the name of a broker, dealer, custodian bank, or other nominee, you should instruct your broker, dealer, custodian bank, or other nominee to exercise your Subscription Rights on your behalf. Please follow the instructions of your nominee, who may require that you meet a deadline earlier than 5:00 PM Eastern Time, on , 2016.
~~Delivery of shares and Warrants~~		As soon as practicable after the expiration of the Rights Offering, the Subscription Agent will arrange for the issuance of the shares of common stock and Warrants purchased pursuant to the Rights Offering. All shares and Warrants that are purchased in the Rights Offeringoffering will be ~~issued~~approximately $13.4 million ($15.4 million if the underwriter’s option to purchase additional Units is exercised in book-entry, or uncertificated, form meaning that you will receive a direct registration (DRS) account statement from our transfer agent reflecting ownership of these securities if you are a holder of record of shares or warrants. If you hold your shares in the name of a custodian bank, broker, dealer, or other nominee, DTC will credit your account with your nominee with the securities you purchased in the Rights Offering.
~~Non-transferability of Subscription Rights~~		~~The Subscription Rights may not be sold, transferred, assigned or given away to anyone. The Subscription Rights will not be listed for trading on any stock exchange or market.~~


~~Transferability of Warrants~~		~~The Warrants will be separately transferable following their issuance and through their expiration years from the issuance date.~~
~~No board recommendation~~		~~Our board of directors is not making a recommendation regarding your exercise of the Subscription Rights. You are urged to make your decision to invest~~full), based on ~~your own assessment~~an assumed public offering price per Unit of ~~our business~~$0.56, the last reported sale price of Common Stock on the Nasdaq Capital Market on April 20, 2018, and ~~the Rights Offering. Please see "Risk Factors" for a discussion of some of the risks involved in investing in our securities.~~
~~No revocation~~		~~All exercises of Subscription Rights are irrevocable, even if you later learn of information that you consider to be unfavorable to the exercise of your Subscription Rights.~~
~~Use of proceeds~~		after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us. We intend to use the net proceeds from this ~~Rights Offering~~offering to ~~continue funding INSPIRE, our ongoing Phase 3 clinical trial of rigosertib IV in a population of patients with higher-risk MDS after failure of HMA therapy, and, to a lesser extent, for other~~fund the development of our clinical and preclinical programs, for other research and development activities ~~business development~~ and for general corporate purposes, which may include capital expenditures and funding ~~our~~ working capital needs. See ~~"Use~~“Use of ~~Proceeds."~~Proceeds” on page 16.
~~Material U.S. federal income tax consequences~~		For U.S. federal income tax purposes, we do not believe you should recognize income or loss upon receipt or exercise of a Subscription Right. You should consult your own tax advisor as to the tax consequences of the Rights Offering in light of your particular circumstances. See "Material U.S. Federal Income Tax Consequences."
~~Extension and termination~~Risk factors		~~Although we do not presently intend to do so, we may extend~~You should read the ~~Rights Offering for additional time in our sole discretion. Our board~~“Risk Factors” section of ~~directors may for any reason terminate the Rights Offering at any time before the completion of the Rights Offering.~~
~~Subscription Agent~~		~~Wells Fargo Bank, N.A.~~
~~Questions~~		If you have any questions about the Rights Offering, please contact the dealer-manager, Maxim Group LLC, at 405 Lexington Avenue, New York, New York 10174, Attention Syndicate Department, email: syndicate@maximgrp.com or telephone: (212) 895-3745.
~~Market for common stock~~		~~Our common stock is listed on NASDAQ under the symbol "ONTX." See "Market Price of our Common Stock~~this prospectus and ~~Related Stockholder Matters."~~


~~Risk factors~~		~~Before you exercise your Subscription Rights to purchase Units, you should be aware that there are risks associated with your investment, and you should carefully read and consider risks described~~ in the ~~section captioned "Risk Factors" together with all of the other information included in~~documents incorporated by reference into this ~~prospectus.~~
~~Dealer-Manager~~		~~Maxim Group LLC.~~
~~Distribution arrangements~~		Under the terms and subject to the conditions contained in the dealer-manager agreement, the dealer-manager will use its best efforts to solicit the exercise of Subscription Rights. We have agreed to pay the dealer-manager certain fees for acting as dealer-manager and to reimburse the dealer-manager for certain out-of-pocket expenses incurred in connection with this offering. The dealer-manager is not underwriting or placing any of the Subscription Rights or the Units, shares of common stock or Warrants being issued in this offering, and do not make any recommendation with respect to such Subscription Rights (including with respect to the exercise or expiration of such Subscription Rights), Units, shares of common stock or Warrants. See "Plan of Distribution"prospectus for a discussion of factors to consider before deciding to invest in our securities.

Listing		Common Stock is listed on the ~~fees~~Nasdaq Capital Market under the symbol “ONTX.” We do not intend to apply for listing of the Pre-Funded Warrants, the Warrants or Series B Preferred Stock on any securities exchange or other nationally recognized trading system. There is no established public trading market for the Pre-Funded Warrants, the Warrants or Series B Preferred Stock, and ~~expenses~~we do not expect a market to ~~be paid to the dealer-manager.~~develop.


	Amount to be Paid		Amount to be Paid
SEC Registration Fee	$	2,266	$	4,429
FINRA Filing Fee		5,000	5,837
NASDAQ Fee		50,000
Nasdaq Fee			5,000
Printing Expenses		40,000	30,000
Legal Fees and Expenses		200,000	150,000
Accounting Fees and Expenses		75,000	53,000
Subscription Agent and Warrant Agent Fees and Expenses		100,000
Transfer Agent Fees and Expenses			20,000
Miscellaneous Expenses		20,000	20,452

	$	492,266



Total			$	288,718

Exhibit
Number	~~ONCONOVA THERAPEUTICS, INC.~~
	~~By:~~	~~/s/ RAMESH KUMAR, PH.D~~ ~~Ramesh Kumar, Ph.D~~ ~~President and Chief Executive Officer~~

~~Signature~~	~~Title~~	~~Date~~


~~/s/ RAMESH KUMAR, PH.D~~ ~~Ramesh Kumar, Ph.D.~~	~~Director, President and Chief Executive Officer (Principal Executive Officer and Principal Operating Officer)~~	~~June 1, 2016~~
~~/s/ MARK GUERIN~~ ~~Mark Guerin~~	~~Vice President, Financial Planning and Accounting (Principal Financial Officer and Principal Accounting Officer)~~	~~June 1, 2016~~
~~/s/ HENRY S. BIENEN, PH.D~~ ~~Henry S. Bienen, Ph.D.~~	~~Director~~	~~June 1, 2016~~
~~/s/ JEROME E. GROOPMAN, M.D.~~ ~~Jerome E. Groopman, M.D.~~	~~Director~~	~~June 1, 2016~~

~~Signature~~	~~Title~~	~~Date~~


~~/s/ MICHAEL B. HOFFMAN~~ ~~Michael B. Hoffman~~	~~Chairman, Board of Directors~~	~~June 1, 2016~~
~~/s/ JAMES J. MARINO~~ ~~James J. Marino~~	~~Director~~	~~June 1, 2016~~
~~/s/ VIREN MEHTA~~ ~~Viren Mehta, Pharm.D~~	~~Director~~	~~June 1, 2016~~
~~/s/ E. PREMKUMAR REDDY, PH.D~~ ~~E. Premkumar Reddy, Ph.D~~	~~Director~~	~~June 1, 2016~~
~~/s/ JACK E. STOVER~~ ~~Jack E. Stover~~	~~Director~~	~~June 1, 2016~~

	December 31, 2017 (unaudited)
							Pro Forma
	Actual			Pro Forma			as Adjusted
Cash and cash equivalents	$	4,024,000		$	12,724,000		$	26,075,000

Long-term liabilities	5,864,000			5,864,000			5,864,000

Stockholders’ (deficit) equity:
Preferred stock, $0.01 par value, 5,000,000 authorized, none issued and outstanding, actual, pro forma and pro form as adjusted
Common stock, $0.01 par value, 25,000,000 authorized, 10,771,163 issued and outstanding, actual; 25,000,000 authorized, 19,426,163 shares issued and outstanding, pro forma; 100,000,000 shares authorized, 46,211,877 issued and outstanding, pro forma as adjusted (1)	108,000			194,000			462,000
Additional paid-in capital	350,514,000			359,128,000			372,211,000
Accumulated other comprehensive loss	3,000			3,000			3,000
Accumulated deficit	(362,316,000		)	(362,316,000		)	(362,316,000		)
Total Onconova Therapeutics, Inc. stockholders’ (deficit) equity	(11,691,000		)	(2,991,000		)	10,360,000
Non-controlling interest	830,000			830,000			830,000
Total stockholders’ (deficit) equity	(10,861,000		)	(2,161,000		)	11,190,000

	March 31, 2016 (unaudited)

	Actual			Pro Forma as Adjusted
Cash and cash equivalents	$	16,835,000		$	—
Long-term liabilities		5,181,000
Stockholders' equity*:
Preferred stock, $0.01 par value, 5,000,000 authorized at March 31, 2016 and March 31, 2016 Pro Forma, none issued and outstanding at March 31, 2016		—			—
Common stock, $0.01 par value, 25,000,000 authorized at March 31, 2016 and March 31, 2016 Pro Forma as Adjusted, 2,740,103 shares issued and outstanding at March 31, 2016 and shares issued and outstanding at March 31, 2016 Pro Forma		27,000			—
Additional paid-in capital		331,775,000			—
Accumulated other comprehensive loss		(16,000	)		—
Accumulated deficit		(325,797,000	)		—

Total Onconova Therapeutics, Inc. stockholders' equity		5,989,000			—
Non-controlling interest		830,000			—

Total stockholders' equity		6,819,000			—

Assumed public offering price per Unit				$	0.56
Historical net tangible book value (deficit) per share as of December 31, 2017	$	(1.01	)
Pro forma increase in net tangible book value per share attributable to the Prior Transactions	0.90
Pro forma net tangible book value per share December 31, 2017	(0.11		)
Increase in pro forma net tangible book value per share attributable to investors purchasing in this offering	0.35
Pro forma as adjusted net tangible book value per share as of December 31, 2017 after this offering				0.24
Dilution per share to investors purchasing in this offering				$	0.32


Subscription Price			$
Net tangible book value per share as of March 31, 2016, before Rights Offering	$	2.49
Increase in net tangible book value per share attributable to Rights Offering
Pro forma net tangible book value per share as of March 31, 2016, after giving effect to Rights Offering
Dilution in net tangible book value per share to purchasers			$

				Option	All Other
		Salary	Bonus	Awards	Compensation	Total
Name and Principal Position	Year	($)	($)(1)	($)(2)	($)(3)	($)

Ramesh Kumar, Ph.D.	2017	538,150	—	81,890	23,581	643,621
President and Chief Executive Officer	2016	413,172	254,028	177,729	19,707	864,636

Steven M. Fruchtman, M.D.	2017	436,154	—	49,105	19,315	504,574
Chief Medical Officer and Senior Vice President, Research and Development	2016	421,784	142,800	119,283	7,179	691,046

Manoj Maniar, Ph.D.	2017	388,977	—	36,108	14,410	439,495
Senior Vice President, Product Development	2016	371,453	126,186	89,703	11,930	599,272

	High		Low

*Year Ending December 31, 2016*
First Quarter	$	10.28	$	3.23
Second Quarter (through June 1, 2016)		6.40		3.80
*Year Ended December 31, 2015*
First Quarter	$	44.30	$	21.50
Second Quarter		30.20		22.60
Third Quarter		40.00		13.20
Fourth Quarter		18.90		9.20
*Year Ended December 31, 2014*
First Quarter	$	162.20		60.50
Second Quarter		64.90		41.00
Third Quarter		57.80		42.40
Fourth Quarter		50.00		32.40

Name	Number of Securities Underlying Unexercised Options (#) Exercisable		Number of Securities Underlying Unexercised Options (#) Unexercisable	Option Exercise Price ($)	Option Expiration Date

Ramesh Kumar	9,376		—	57.60	3/16/2020
	5,251		—	61.30	12/9/2020
	1,033		—	61.30	12/4/2021
	18,754		—	132.80	12/18/2022
	10,500		—	150.00	7/25/2023
	13,500	(1)	—	134.80	12/20/2023
	13,125	(1)	4,375	39.80	12/17/2024
	5,833	(1)	2,917	23.20	4/15/2025
	4,921	(1)	3,829	14.80	9/24/2025
	14,644		—	6.50	1/25/2026
	18,333	(2)	25,667	3.24	9/1/2026
	4,224	(2)	8,449	2.65	12/15/2026
	13,452	(2)	30,575	2.70	1/17/2027

Steven Fruchtman	8,750	(1)	3,250	43.70	1/11/2025
	2,333	(1)	1,167	24.80	4/19/2025
	2,250	(1)	1,750	14.80	9/24/2025
	12,410		—	6.50	1/25/2026
	10,416	(2)	14,584	3.24	9/1/2026
	2,533	(2)	5,066	2.65	12/15/2026
	8,066	(2)	18,335	2.70	1/17/2027

Manoj Maniar	5,625		—	57.60	3/16/2020
	2,625		—	61.30	12/9/2020
	378		—	61.30	12/4/2021
	3,000		—	132.80	12/18/2022
	500		—	150.00	7/25/2023
	4,000	(1)	—	134.80	12/20/2023
	4,500	(1)	1,500	39.80	12/17/2024
	2,666	(1)	1,334	23.20	4/15/2025
	2,250	(1)	1,750	14.80	9/24/2025
	10,340		—	6.50	1/25/2026
	7,083	(2)	9,917	3.24	9/1/2026
	1,862	(2)	3,725	2.65	12/15/2026
	5,931	(2)	13,482	2.70	1/17/2027

Plan Category	Number of Shares to be Issued Upon Exercise of Outstanding Options, Warrants and Rights	Weighted-Average Exercise Price of Outstanding Options, Warrants and Rights		Number of Shares Remaining Available for Future Issuance Under the Equity Compensation Plan (Excluding Shares in First Column)
Equity compensation plans approved by stockholders	894,996	$	40.41	57,632
Equity compensation plans not approved by stockholders	—	—		—

	Number of	Percentage of
	Shares	Shares
	Beneficially	Beneficially
Name and Address of Beneficial Owner	Owned	Owned
5% or greater stockholders:

The Michael and Jane Hoffman 2013	1,377,306	6.9	%
Descendants Trust(1)
712 Fifth Avenue, 51st Fl.
New York, NY 10019

Michael B. Hoffman(2)	1,407,419	7.1	%
(Includes The Michael and Jane
Hoffman 2013 Descendants Trust)
712 Fifth Avenue, 51st Fl
New York, NY 10019

Tyndall Capital Partners, L.P.(3)	1.387.646	12.4	%
405 Park Avenue, Suite 1104
New York, NY 10022

683 Capital Partners, LP, 683 Capital Management, LLC,	2,664,934	14.4	%
and Ari Zweiman(14)
3 Columbus Circle, Suite 2205
New York, NY 10019

Sabby Healthcare Master Fund, Ltd.,
Sabby Volatility Warrant Master Fund, Ltd., Sabby Management, LLC and Hal Mintz (15)	561,163	5.21	%

Sabby Healthcare Master Fund, Ltd.
c/o Ogier Fiduciary Services (Cayman) Limited
89 Nexus Way, Camana Bay
Grand Cayman KY1-9007
Cayman Islands

Sabby Volatility Warrant Master Fund, Ltd.
c/o Ogier Fiduciary Services (Cayman) Limited
89 Nexus Way, Camana Bay

Grand Cayman KY1-9007
Cayman Islands

Sabby Management, LLC
10 Mountainview Road, Suite 205
Upper Saddle River, New Jersey 07458

Hal Mintz
c/o Sabby Management, LLC
10 Mountainview Road, Suite 205
Upper Saddle River, New Jersey 07458

Armistice Capital, LLC, Armistice Capital Master Fund Ltd., and	1,607,500	9.0	%
Steven Boyd(16)
Armistice Capital, LLC
510 Madison Avenue, 22nd Floor
New York, New York 10022

Armistice Capital Master Fund Ltd.
c/o dms Corporate Services Ltd.
20 Genesis Close
P.O. Box 314
Grand Cayman KY1-1104
Cayman Islands

Steven Boyd
c/o Armistice Capital, LLC
510 Madison Avenue, 22nd Floor
New York, New York 10022

Other Directors, Director Nominees and Named Executive Officers:
Henry S. Bienen, Ph.D.(5)	27,438	*
Jerome E. Groopman, M.D.(6)	10,985	*
Ramesh Kumar, Ph.D.(7)	371,304	1.9	%
Manoj Maniar, Ph.D.(8)	61,280	*
James J. Marino(9)	158,969	*
Steven M. Fruchtman, M.D.(10)	72,911	*
Viren Mehta(11)	19,877	*
E. Premkumar Reddy, Ph.D.(4)	492,832	2.5	%
Jack E. Stover(12)	3,785	*
All current executive officers, directors and director nominees as a group (11 persons)(13)	2,675,159	13.1	%

			Total
	Per Unit	Per Pre-Funded Unit	Without Option Exercise	With Full Option Exercise
Public offering price
Underwriting discounts and commissions
Proceeds, before expenses, to us

~~Exhibit~~ ~~Number~~			Exhibit Description

1.1†	~~3.1~~	Form of Underwriting Agreement

3.1		Tenth Amended and Restated Certificate of Incorporation of Onconova Therapeutics, Inc.(Incorporated by reference to Exhibit 3.1 to the ~~Company's~~Company’s Current Report on Form 8-K filed on July ~~25,~~30, 2013).

3.2	~~3.2~~		Amended and Restated Bylaws of Onconova Therapeutics, Inc.(Incorporated by reference to Exhibit 3.1 to the ~~Company's~~Company’s Current Report on Form 8-K filed on July ~~25,~~30, 2013).

3.3	~~3.3~~		Certificate of Amendment to Tenth Amended and Restated Certificate of Incorporation of Onconova Therapeutics, Inc.(Incorporated by reference to Exhibit 3.1 to the ~~Company's~~Company’s Current Report on Form 8-K filed on May 31, 2016).

3.4	~~4.1~~	Certificate of Designation of Series A Convertible Preferred Stock (Incorporated by reference to Exhibit 3.1 to the Company’s Current Report on Form 8-K filed on February 8, 2018).

3.5		Certificate of Amendment to Tenth Amended and Restated Certificate of Incorporation of Onconova Therapeutics, Inc., as amended (Incorporated by reference to Exhibit 3.1 to the Company’s Current Report on Form 8-K filed on March 22, 2018).

3.6†		Certificate of Designation of Series B Convertible Preferred Stock

4.1		Form of Certificate of Common Stock(Incorporated by reference to Exhibit 4.1 to Pre-Effective Amendment No. 1 the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on July 11, ~~2013.)~~2013).

4.2	~~4.2~~		Eighth Amended and Restated ~~Stockholders'~~Stockholders’ Agreement, effective as of July 27, 2012, by and among Onconova Therapeutics, Inc. and certain stockholders named therein(Incorporated by reference to Exhibit 4.2 to Pre-Effective Amendment No. 1 to the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on July 11, 2013).

4.3	~~4.3~~		Amendment No. 1 to Eighth Amended and Restated ~~Stockholders'~~Stockholders’ Agreement, effective as of July 9, 2013(Incorporated by reference to Exhibit 4.2 to Pre-Effective Amendment No. 1 the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on July 11, 2013).

4.4	~~4.4~~		Form of ~~warrant~~Warrant Certificate issued ~~January 11,~~pursuant to Warrant Agreement, dated as of July 27, 2016, by and between Onconova Therapeutics, Inc. and Wells Fargo Bank, N.A., as Warrant Agent (Incorporated by reference to Exhibit 4.1 to the ~~Company's~~Company’s Quarterly Report on Form 10-Q filed on August 15, 2016).

4.5		Warrant Agreement, dated as of July 27, 2016, by and between Onconova Therapeutics, Inc. and Wells Fargo Bank, N.A., as Warrant Agent (Incorporated by reference to Exhibit 4.2 to the Company’s Quarterly Report on Form 10-Q filed on August 15, 2016).

4.6		Form of Pre-Funded Warrants issued as of July 27, 2016 (Incorporated by reference to Exhibit 4.3 to the Company’s Quarterly Report on Form 10-Q filed on August 15, 2016).

4.7		Form of Underwriter Warrant issued as of February 12, 2018 (Incorporated by reference to Exhibit 4.1 to the Company’s Current Report on Form 8-K filed on ~~January 6, 2016)~~February 8, 2018).

4.8	~~4.5~~		Form of ~~Subscription Rights Statement~~Preferred Stock Warrant issued as of February 12, 2018 (Incorporated by reference to Exhibit 4.2 to the Company’s Current Report on Form 8-K filed on February 8, 2018).

4.9	~~4.6~~	† Form of Pre-Funded Warrant issued as of February 12, 2018 (Incorporated by reference to Exhibit 4.3 to the Company’s Current Report on Form 8-K filed on February 8, 2018).

4.10†		Form of Warrant ~~Certificate for Warrants underlying Units~~

4.11†	~~4.7~~	†	Form of Pre-Funded Warrant ~~Agreement~~

4.12†	~~5.1~~	† Form of Underwriter Warrant

5.1		Opinion of ~~Pepper Hamilton~~Morgan, Lewis & Bockius LLP

10.1*	~~10.1~~	*	Development and License Agreement, effective as of September 19, 2012, by and between Onconova Therapeutics, Inc. and Baxter Healthcare SA(Incorporated by reference to Exhibit 10.1 to Pre-Effective Amendment No. 2 the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on July 18, 2013).

10.2*	~~10.2~~	*	License Agreement, effective as of July 5, 2011, by and between Onconova Therapeutics, Inc. and SymBio Pharmaceuticals Limited(Incorporated by reference to Exhibit 10.2 to Pre-Effective Amendment No. 2 the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on July 18, 2013).

10.3*	~~10.3~~	*	First Amendment to License Agreement, effective as of September 2, 2011, by and between Onconova Therapeutics, Inc. and SymBio Pharmaceuticals Limited(Incorporated by reference to Exhibit 10.3 to the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on June 14, 2013).

10.4*	~~10.4~~	*	License Agreement, effective as of January 1, 1999, by and between Onconova Therapeutics, Inc. and Temple University—Of The Commonwealth System of Higher Education(Incorporated by reference to Exhibit 10.4 to the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on June 14, 2013).

10.5*	~~10.5~~	*	Amendment to License Agreement, effective as of September 1, 2000, by and between Temple University—Of The Commonwealth System of Higher Education and Onconova Therapeutics, Inc.(Incorporated by reference to Exhibit 10.5 to the ~~Company's~~Company’s Registration Statement on Form S-1 filed on June 14, 2013).

10.6*

~~Exhibit~~ ~~Number~~			~~Exhibit Description~~

	~~10.6~~	*	Amendments to Exclusive License Agreement, effective as of March 21, 2013, by and between Temple University—Of The Commonwealth System of Higher Education and Onconova Therapeutics, Inc.(Incorporated by reference to Exhibit 10.6 to the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on June 14, 2013).

10.7*	~~10.7~~	*	~~Definitive Agreement, effective as of May 12, 2010, by and between Onconova Therapeutics, Inc. and The Leukemia and Lymphoma Society~~~~(Incorporated by reference to Exhibit 10.7 to the Company's Registration Statement on Form S-l filed on June 14, 2013).~~

	~~10.8~~	*	~~First Amendment to Definitive Agreement, effective as of June 23, 2011, by and between Onconova Therapeutics, Inc. and The Leukemia and Lymphoma Society~~~~(Incorporated by reference to Exhibit 10.8 to the Company's Registration Statement on Form S-l filed on June 14, 2013).~~

	~~10.9~~	*	~~Second Amendment to Definitive Agreement, effective as of May 29, 2012, by and between Onconova Therapeutics, Inc. and The Leukemia and Lymphoma Society~~~~(Incorporated by reference to Exhibit 10.9 to the Company's Registration Statement on Form S-l filed on June 14, 2013).~~

	~~10.10~~	*	~~Third Amendment to Definitive Agreement, effective as of January 5, 2013, by and between Onconova Therapeutics, Inc. and The Leukemia and Lymphoma Society~~~~(Incorporated by reference to Exhibit 10.10 to the Company's Registration Statement on Form S-l filed on June 14, 2013).~~

	~~10.11~~	*	~~Termination of Agreement, effective as of February 5, 2013, by and between Onconova Therapeutics, Inc. and The Leukemia and Lymphoma Society~~~~(Incorporated by reference to Exhibit 10.11 to the Company's Registration Statement on Form S-l filed on June 14, 2013).~~

	~~10.12~~	*	Limited Liability Company Agreement of GBO, LLC, dated as of December 12, 2012, by and between Onconova Therapeutics, Inc. and GVK Biosciences Private Limited(Incorporated by reference to Exhibit 10.12 to the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on June 14, 2013).

10.8	~~10.13~~		Onconova Therapeutics, Inc. 2007 Equity Compensation Plan, and forms of agreement thereunder(Incorporated by reference to Exhibit 10.13 to Pre-Effective Amendment No. 1 the ~~Company's~~Company’s Registration Statement on Form ~~S-l~~S-1 filed on July 11, 2013).

10.9	~~10.14~~		Employment Agreement, effective as of July 1, 2015, by and between Onconova Therapeutics, Inc. and Ramesh Kumar, Ph.D.(Incorporated by reference to Exhibit 10.1 to the ~~Company's~~Company’s Current Report on Form 8-K filed on July 8, 2015).

10.10	~~10.15~~		Letter Agreement, effective as of January 1, 2016, by and between Onconova Therapeutics, Inc. and Ramesh Kumar, Ph.D.(Incorporated by reference to Exhibit 10.2 to the ~~Company's~~Company’s Current Report on Form 8-K filed on February 17, 2016).

10.11	~~10.16~~		Amended and Restated Employment Agreement, effective as of July 1, 2015, by and between Onconova Therapeutics, Inc. and Thomas McKearn, M.D., Ph.D.(Incorporated by reference to

		Exhibit 10.2 to the ~~Company's~~Company’s Current Report on Form 8-K filed on July 8, 2015).

10.12	~~10.17~~		Amended and Restated Employment Agreement, effective as of July 1, 2015, by and between Onconova Therapeutics, Inc. and Ajay Bansal.(Incorporated by reference to Exhibit 10.4 to the ~~Company's~~Company’s Current Report on Form 8-K filed on July 8, 2015).

10.13	~~10.18~~		Consulting Agreement, effective as of January 1, 2012, by and between Onconova Therapeutics, Inc. and E. Premkumar Reddy, Ph.D., including Consultant Agreement Renewal, dated February 27, 2013(Incorporated by reference to Exhibit 10.23 to the ~~Company's~~Company’s Registration Statement on Form S-1 filed on June 14, 2013).

~~Exhibit~~ ~~Number~~			~~Exhibit Description~~


	~~10.19~~
10.14		Form of Indemnification Agreement entered into by and between Onconova Therapeutics, Inc. and each director and executive officer(Incorporated by reference to Exhibit 10.24 to Pre-Effective Amendment No. 1 the ~~Company's~~Company’s Registration Statement on Form S-1 filed on July 11, 2013).

10.15	~~10.20~~		Onconova Therapeutics, Inc. 2013 Equity Compensation Plan, and forms of agreement thereunder(Incorporated by reference to Exhibit 10.25 to Pre-Effective Amendment No. 1 the ~~Company's~~Company’s Registration Statement on Form S-1 filed on July 11, 2013).

10.16	~~10.21~~		Onconova Therapeutics, Inc. 2013 Performance Bonus Plan(Incorporated by reference to Exhibit 10.26 to Pre-Effective Amendment No. 1 the ~~Company's~~Company’s Registration Statement on Form S-1 filed on July 11, 2013).

10.17	~~10.22~~		Employment Agreement, effective as of July 1, 2015, by and between Onconova Therapeutics, Inc. and ~~Dr.Manoj~~Dr. Manoj Manair(Incorporated by reference to Exhibit 10.3 to the ~~Company's~~Company’s Current Report on Form 8-K filed on July 8, 2015).

10.18	~~10.23~~		Employment Agreement, effective as of July 1, 2015, by and between Onconova Therapeutics, Inc. and Mark Guerin(Incorporated by reference to Exhibit 10.2 to the ~~Company's~~Company’s Current Report on Form 8-K filed on February 17, 2016).

10.19	~~10.24~~		Amended and Restated Employment Agreement, effective as of July 1, 2015, by and between Onconova Therapeutics, Inc. and Steven M. Fruchtman, M.D.(Incorporated by reference to Exhibit 10.5 to the ~~Company's~~Company’s Quarterly Report on Form 10-Q filed on August 13, 2015).

10.20	~~10.25~~		Purchase Agreement between Onconova Therapeutics, Inc. and Lincoln Park Capital Fund, LLC, dated October 8, 2015(Incorporated by reference to Exhibit 10.1 to the ~~Company's~~Company’s Current Report on Form 8-K filed on October 8, 2015).

10.21	~~10.26~~		~~Registration Rights~~Dealer-Manager Agreement dated July 7, 2016, between Onconova Therapeutics, Inc. and ~~Lincoln Park Capital Fund,~~Maxim Group LLC ~~dated October 8, 2015~~(~~Incorporated by reference to Exhibit 10.2 to the Company's Current Report on Form 8-K filed on October 8, 2015).~~

	~~10.27~~		~~Form of Securities Purchase Agreement between Onconova Therapeutics and the Investor party thereto, dated January 5, 2016 (~~Incorporated by reference to Exhibit 10.1 to the ~~Company's~~Company’s Current Report on Form 8-K filed on ~~January 6,~~July 13, 2016).

10.22	~~21.1~~	At Market Issuance Sales Agreement, dated December 5, 2016, between Onconova Therapeutics, Inc. and FBR Capital Markets & Co. (Incorporated by reference to Exhibit 10.1 to the Company’s Current Report on Form 8-K filed on December 5, 2016).

10.23		Letter Agreement, effective as of January 1, 2017, by and between Onconova Therapeutics, Inc. and Ramesh Kumar, Ph.D. (Incorporated by reference to Exhibit 10.27 to the Company’s Annual Report on Form 10-K filed on March 29,2017).

21.1		Subsidiaries of Onconova Therapeutics, Inc.(Incorporated by reference to Exhibit 21.1 to the ~~Company's~~Company’s Annual Report on Form 10-K filed on March ~~28,2016)~~16,2018).

23.1	~~23.1~~		Consent of Ernst & Young, LLP.

	~~23.2~~		~~Consent of Pepper Hamilton LLP (included in Exhibit 5.1)~~

	~~24.1~~		~~Power of Attorney (included on signature page)~~

	~~99.1~~		~~Form of Instructions as to Use of Subscription Rights Statements~~

	~~99.2~~		~~Form of Letter to Shareholders who are Record Holders~~

	~~99.3~~		~~Form of Letter to Brokers, Dealers, Banks and Other Nominees~~

	~~99.4~~		~~Form of Broker Letter to Clients Who are Beneficial Holders~~

	~~99.5~~		~~Form of Beneficial Owner Election Form~~

	~~99.6~~		~~Form of Nominee Holder Certification~~

23.2		Consent of Morgan, Lewis & Bockius LLP (included in Exhibit 5.1).
~~Exhibit~~ ~~Number~~		~~Exhibit Description~~
24.1**	~~99.7~~	~~Form~~ Power of ~~Notice of Important Tax Information~~Attorney (included on the signature page).

Onconova Therapeutics, Inc.

By:	/s/ RAMESH KUMAR, PH.D
	Name:	Ramesh Kumar, Ph.D
	Title:	President and Chief Executive Officer

Name		Title

/s/ RAMESH KUMAR, PH.D.		Director, President and Chief Executive Officer (Principal
Ramesh Kumar, Ph.D.		Executive Officer)

/s/ MARK GUERIN		Chief Financial Officer (Principal Financial Officer and Principal
Mark Guerin		Accounting Officer)

*		Director
Henry S. Bienen, Ph.D.

*		Director
Jerome E. Groopman, M.D.

*		Chairman, Board of Directors
Michael B. Hoffman

*		Director
James J. Marino

*		Director
Viren Mehta, Pharm.D

*		Director
E. Premkumar Reddy, Ph.D

*		Director
Jack E. Stover

*By:	/s/ Ramesh Kumar, Ph.D.
	Name: Ramesh Kumar, Ph.D., Attorney-in-Fact