Exhibit 5.3

August 14, 2020

Omeros Corporation

201 Elliott Avenue West

Seattle, Washington 98119

Ladies and Gentlemen:

We have acted as special counsel to Omeros Corporation, a Washington corporation (the “Company”), in connection with the offering by the Company under the Securities Act of 1933 (the “Securities Act”) of up to $241,500,000 in aggregate principal amount (including up to $31,500,000 aggregate principal amount issuable upon the exercise of the underwriters’ option to purchase additional notes solely to cover over-allotments) of the Company’s 5.25% Convertible Senior Notes due 2026 (the “Notes”) pursuant to the Indenture, dated August 14, 2020 (the “Indenture”), between the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”), pursuant to the registration statement on Form S-3 (File No. 333-235349), filed with the Securities and Exchange Commission (the “Commission”) on December 4, 2019 (the “Registration Statement”).

We have reviewed:

We also have reviewed such corporate records, certificates and other documents, and such questions of law, as we have considered necessary or appropriate for the purposes of this opinion. We have assumed that all signatures are genuine, that all documents submitted to us as originals are authentic and that all copies of documents submitted to us conform to the originals.

We have assumed further that the Company has duly authorized the Notes and that it has duly authorized, executed and delivered the Indenture. We have assumed further that the Company is a corporation duly organized, validly existing and in good standing under the laws of the State of Washington and has all legal right, power, and authority to issue the Notes and to execute, deliver and perform its obligations under the Indenture and the Notes. We note that you are relying with respect to all matters of Washington law on an opinion of Keller Rohrback, L.L.P., dated as of the date hereof, which opinion is filed as Exhibit 5.2 to the Current Report on Form 8-K that will be incorporated by reference into the Registration Statement.

Additionally, we have relied as to certain matters on information obtained from public officials, officers of the Company and other sources believed by us to be responsible.

Based on the foregoing and subject to the qualifications set forth herein, we are of the opinion that, when the Notes have been (a) duly executed by the Company and duly authenticated and delivered by the Trustee in accordance with the Indenture and (b) issued and delivered by the Company against payment of the purchase price therefor in accordance with the Underwriting Agreement, the Notes will constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their terms, subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and other laws of general applicability relating to or affecting creditors’ rights and to general equity principles.

The foregoing opinion is subject to the following qualifications. We express no opinion as to: (i) waivers of defenses, subrogation and related rights, rights to trial by jury, rights to object to venue, or other rights or benefits bestowed by operation of law; (ii) releases or waivers of unmatured claims or rights; (iii) indemnification, contribution, or exculpation provisions, or provisions for the non-survival of representations, to the extent they purport to indemnify any party against, or release or limit any party’s liability for, its own breach or failure to comply with statutory obligations, or to the extent such provisions are contrary to public policy; (iv) provisions for liquidated damages and penalties, penalty interest and interest on interest; (v) provisions purporting to supersede equitable principles, including provisions requiring amendments and waivers to be in writing and provisions making notices effective even

if not actually received; (vi) restrictions upon transfers, pledges or assignments of a party’s rights under the Indenture; (vii) provisions purporting to make a party’s determination conclusive or permitting a party to act in its sole or absolute discretion; or (viii) exclusive jurisdiction or venue provisions.

We are members of the bar of the District of Columbia and the State of New York. We do not express any opinion herein on any laws other than the laws of the State of New York.

We hereby consent to the filing of this opinion as Exhibit 5.3 to the Current Report on Form 8-K that will be incorporated by reference into the Registration Statement. We also hereby consent to the reference to our firm under the heading “Legal Matters” in the prospectus constituting part of the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.