As of January 1, 2021, all sites and article galleys published after January 1, 2012 must comply with the Accessibility for Ontarians with Disabilities Act (AODA).
AODA requirements for websites are based on the Web Content Accessibility Guidelines (WCAG) version 2.0, level AA. You can find a quick reference guide called How to Meet WCAG online but you may find that this is overly technical and complex for your purposes: WCAG covers content types and technologies not used with OJS, and many WCAG requirements are covered by the OJS platform itself.
The best starting point for making accessible, AODA compliant content in OJS is PKP’s Creating Accessible Content: A Guide for Journal Editors and Authors guide. Although the guide does not cite AODA, it is based on the WCAG standards used by AODA. The guide is most applicable to article galleys, but the same principles apply to content on the OJS site itself.
You should also refer to the Inclusive and Accessible Theming guide to understand how to keep make and keep your OJS pages accessible.
York Digital Journals - the York University Libraries program that provides journal hosting services.