Update On Information Relating To The General Meeting Of September 30, 2025

61 Founding, Historical, Employee and Executive Shareholders of the Company, Representing Approximately 20.1% of Voting Rights have entered into a Shareholders' Agreement

NANTES, France – September 12, 2025, 8:00 a.m. – OSE Immunotherapeutics SA (ISIN: FR0012127173; Mnemo: OSE) has been informed of the execution, on September 11, 2025, of a shareholders' agreement (the“ Agreement ”) between 61 founding, historical, employee and executive shareholders of the Company (the“ Shareholders' Group ”), relating to the vote on resolutions concerning the composition of the Board of Directors at the general meeting of September 30, 2025 (the“ General Meeting ”).

The Shareholders' Group, acting in concert, directly and indirectly holds approximately 15.7% of the share capital and approximately 20.1% of the voting rights of the Company. In this context, the Company has been informed that a declaration of threshold crossing and intent has been submitted to the AMF.

The Shareholders' Group has committed to vote at the General Meeting: in favor of the resolutions proposed by the Board of Directors concerning its own composition (resolutions 5 to 8); and against the resolutions proposed by the group of minority shareholders acting in concert (resolutions A to K)¹; it being specified that voting rights may be exercised freely for the remaining resolutions.

Terms of Participation in the General Meeting
As a reminder, shareholders may participate in the General Meeting:

• Either in person.
• Or by voting by mail or online via VOTACCESS.
• Or by being represented by granting proxy to the Chairman.
• Or by being represented by granting proxy to any other person of their choice (Articles L.225-106 and L.22-10-39 of the French Commercial Code).

It is specified that for any proxy granted by a shareholder without naming a proxyholder, the Chairman of the General Meeting will vote in favor of the resolutions presented or approved by the Board of Directors and against all other resolutions.

To cast any other vote, the shareholder must appoint a proxyholder who agrees to vote in accordance with the instructions given by the principal.

Shareholders are invited to consult the documents available on the Company's website:

For any questions regarding the Annual General Meeting or to obtain assistance with voting procedures, shareholders may contact the Company at:
... and / or using the toll-free number: 0 805 650 064 (available Monday to Friday, from 9:00 a.m. to 6:00 p.m., Paris time).

ABOUT OSE IMMUNOTHERAPEUTICS

OSE Immunotherapeutics is a biotech company dedicated to developing first-in-class assets in immuno-oncology (IO) and immuno-inflammation (I&I) that address the unmet patient needs of today and tomorrow. We partner with leading academic institutions and biopharmaceutical companies in our efforts to develop and bring to the market transformative medicines for people with serious diseases. OSE Immunotherapeutics is based between Nantes and Paris and is quoted on Euronext.

Additional information about OSE Immunotherapeutics assets is available on the Company's website: . Follow us on LinkedIn.

Contacts

Forward-looking statements
This press release contains express or implied information and statements that might be deemed forward-looking information and statements in respect of OSE Immunotherapeutics. They do not constitute historical facts. These information and statements include financial projections that are based upon certain assumptions and assessments made by OSE Immunotherapeutics' management considering its experience and its perception of historical trends, current economic and industry conditions, expected future developments and other factors they believe to be appropriate.

These forward-looking statements include statements typically using conditional and containing verbs such as“expect”,“anticipate”,“believe”,“target”,“plan”, or“estimate”, their declensions and conjugations and words of similar import. Although the OSE Immunotherapeutics management believes that the forward-looking statements and information are reasonable, the OSE Immunotherapeutics' shareholders and other investors are cautioned that the completion of such expectations is by nature subject to various risks, known or not, and uncertainties which are difficult to predict and generally beyond the control of OSE Immunotherapeutics. These risks could cause actual results and developments to differ materially from those expressed in or implied or projected by the forward-looking statements. These risks include those discussed or identified in the public filings made by OSE Immunotherapeutics with the AMF. Such forward-looking statements are not guarantees of future performance. This press release includes only summary information and should be read with the OSE Immunotherapeutics Universal Registration Document filed with the AMF on April 30, 2025, including the annual financial report for the fiscal year 2024, available on the OSE Immunotherapeutics' website. Other than as required by applicable law, OSE Immunotherapeutics issues this press release at the date hereof and does not undertake any obligation to update or revise the forward-looking information or statements.

1 Under the terms of a shareholders' agreement published on June 3, 2025, on the AMF website (Declaration No. 225C0906 filed by Ms. Costantini, Mr. Alexis Peyroles, and Mr. Emile Loria).

Attachments

• EN_250912_ShareholderAgreement_VF
• EN_250912_ShareholderAgreement_VF

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