FWIW, if all you're doing is authenticating a web application, you really can just do it off of the OIDC spec. It's a small subset of the protocol and can be easily understood. I did it recently, writing a multi-tenant application where each tenant can have its own OIDC providers, and it took me about three hours to do and to do correctly (verified against Auth0 and Okta, at least).

Barring that, Okta at least is a conformant OIDC implementation and you can use standard libraries for it.