Most “plain” OAuth2 IdPs are likely configurable using Keycloak’s generic OIDC IdP type. Post here which one you’re trying to implement with a link to the developer docs for it, and I’ll take a crack at a configuration.
If that’s doesn’t work, it’s possible to implement your own IdP providers, just as you have noticed that some of the included social identity providers have done. This is a little more work, but is also something that this list can help you with.