May 15! It's on the OptomCAS website:
Applicant Responsibilities & Code of Conduct | OptomCAS
"Respond promptly to all offers of admission. Once you have accepted an offer, it is your obligation to immediately notify and withdraw any outstanding applications or decline offers of admission from other schools and colleges of optometry. If you are holding more than one seat after May 15, failure to decline multiple offers may result in offers of admission being changed or revoked. Applicants who receive multiple offers after May 15th must respond to those offers quickly and failure to do so may result in offers of admission being changed or revoked. Schools and colleges of optometry participate in Shared Acceptance Reports during the OptomCAS cycle which allows them to be informed when candidates are holding more than one seat."