奥麦罗制药美股招股说明书(2025-07-25版)

5,365,853 Shares of Common Stock We are offering 5,365,853 shares of our common stock, par value $0.01 per share, directly to aninstitutional investor pursuant to this prospectus supplement and the accompanying prospectus, at apurchase price equal to $4.10 per share. Our common stock is traded on The Nasdaq Global Market under the symbol “OMER.” On July24,2025, the last reported sale price of our common stock on The Nasdaq Global Market was $3.59 per share. We have engaged D. Boral Capital LLC (the “Placement Agent”) to act as our exclusive placementagent in connection with this offering. We have agreed to pay to the Placement Agent the fees set forth inthe table below. We will bear all costs associated with the offering. See “Plan of Distribution” beginning onpageS-11of this prospectus supplement for more information regarding these arrangements. Investing in our common stock involves significant risks. Before buying shares of our common stock, youshould carefully consider the risks described under the caption “Risk Factors” beginning on pageS-6of thisprospectus supplement and in the documents incorporated by reference into this prospectus supplement. Neither the Securities and Exchange Commission (the “SEC”) nor any state securities commission hasapproved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectussupplement or the accompanying prospectus. Any representation to the contrary is a criminal offense. (1)We have agreed to pay the Placement Agent a fee equal to 6% of the gross proceeds of this offering.See “Plan of Distribution” beginning on pageS-11for additional information for a description of thecompensation payable to the Placement Agent. Delivery of the common stock being offered pursuant to this prospectus supplement and theaccompanying prospectus is expected to occur on or about July28, 2025. The date of this prospectus supplement is July24, 2025 TABLE OF CONTENTS Prospectus Supplement PageAbout this Prospectus SupplementS-iiForward-Looking StatementsS-iiiProspectus Supplement SummaryS-1The OfferingS-5Risk FactorsS-6Use of ProceedsS-9DilutionS-10Plan of DistributionS-11Legal MattersS-12ExpertsS-12Where You Can Find More InformationS-12Incorporation By ReferenceS-13 Prospectus PageAbout this ProspectusiiForward-Looking StatementsiiiAbout Omeros1Where You Can Find More Information2Incorporation by Reference2Risk Factors3Use of Proceeds4Description of Common Stock5Description of Preferred Stock8Description of Debt Securities10Description of Depository Shares20Description of Warrants23Description of Subscription Rights24Description of Units25Plan of Distribution26Legal Matters29Experts29 ABOUT THIS PROSPECTUS SUPPLEMENT This document is in two parts. The first part is this prospectus supplement, which describes the specificterms of this offering of common stock and updates the information contained in the accompanyingprospectus and the documents incorporated by reference herein and therein. The second part is theaccompanying prospectus, which provides more general information, some of which does not apply to thisoffering. If information included or incorporated by reference in this prospectus supplement is inconsistentwith the accompanying prospectus or the information incorporated by reference therein, then this prospectussupplement or the information incorporated by reference in this prospectus supplement will apply and willsupersede the information in the accompanying prospectus and the documents incorporated by referencetherein. For a more detailed understanding of our common stock, you should read both this prospectussupplement and the accompanying prospectus, together with additional information described under theheading “Where You Can Find More Information.” This prospectus supplement is part of a shelf registration statement on Form S-3 (File No. 333-268269)that we filed with the SEC on November9, 2022, and was declared effective by the SEC on November17,2022. Under the shelf registration process, we may from time to time offer and sell any combination of thesecurities described in the accompanying prospectus in one or more offerings. Neither we nor the Placement Agent have authorized anyone to provide you with information that isdifferent or in addition to that contained or incorporated by reference in this prospectus supplement, theaccompanying prospectus or any free writing prospectus prepared by us or on our behalf. Neither we nor thePlacement Agent take any responsibility for, and can provide no assurance as to the reliability of, anyinformation that others may provide. Neither we nor the Placement Agent are making an offer to sell orsoliciting an offer to buy our common stock under any circumstance in any jurisdiction where the offer orsolicitation is not permitted. You should assume that the information contained in this prospectus supplement,the accompanying prospectus and any free writing prospectus prepared by us or on our