There are several open access publishing models, the main ones are Green, Gold and Diamond (all described below). Not expanded on here are Hybrid (which charges APCs and fees to access for the readers/institutions, not a sustainable model), Bronze (free to read but difficult to reuse as licenses are not identifiable) and Black (unauthorised sharing of articles kept behind paywalls)
Gold, Green and Diamond explained
(source unknown, 9https://guides.library.cmu.edu/openaccess)
In the gold OA model, the publisher makes all articles and related content available for free immediately on the journal's website. In such publications, articles are licensed for sharing and reuse via Creative Commons licenses or similar. No subscription fees are charged for any part of a journal that is fully gold open access.
Many gold OA publishers charge an article processing charge (APC), which is typically paid through institutional or grant funding. The majority of gold open access journals charging APCs follow an "author-pays" model, although this is not an intrinsic property of gold OA.