Osisko Development Corp. — Capital/Financing Update 2026

Jan 28, 2026

Cassels

FILED BY SEDAR+

January 27, 2026

• British Columbia Securities Commission
• Alberta Securities Commission
• Financial and Consumer Affairs Authority of Saskatchewan
• Manitoba Securities Commission
• Ontario Securities Commission
• Autorité des marchés financiers
• Financial and Consumer Services Commission (New Brunswick)
• Superintendent of Securities, Department of Justice and Public Safety, Prince Edward Island
• Nova Scotia Securities Commission
• Superintendent of Securities, Newfoundland and Labrador
• Office of the Superintendent of Securities Northwest Territories
• Office of the Yukon Superintendent of Securities
• Nunavut Securities Office

Dear Sirs/Mesdames:

RE: OSISKO DEVELOPMENT CORP. (the “Corporation”)
Prospectus Supplement dated January 27, 2026
(the “Supplement”)

We refer to the Supplement of the Corporation in connection with an offering of common shares.

We, as counsel to the Underwriters, hereby consent to the reference to and use of our opinion under the heading “Eligibility for Investment”, and to the reference to our firm name on page iii of the Supplement, under the heading “Legal Matters”, and under the heading “Documents Filed as Part of the Registration Statement”.

We confirm that we have read the Supplement and that we have no reason to believe that there are any misrepresentations in the information contained therein that is derived from our opinion referred to above or that is within our knowledge as a result of the services we performed in connection with such opinion.

Yours truly,

“Cassels Brock & Blackwell LLP”

t: 416 869 5300
f: 416 360 8877
cassels.com
Cassels Brock & Blackwell LLP
Suite 3200, Bay Adelaide Centre – North Tower, 40 Temperance Street
Toronto, ON M5H 0B4 Canada