Comment by mooreds

Yeah, it's a profile on top of OAuth, which leverages aspects (the authorization code grant, tokens) but adds some other functionality (another token with authentication information and some defined claims). I'm not aware of any other profiles with anywhere near the uptake of OIDC.

There are a few folks out there doing pure OAuth, but much of the time it is paired with OIDC. It's pretty darn common to want someone to be authenticated the same time they are authorized, especially in a first party context.

I wrote more on OIDC here: https://fusionauth.io/articles/identity-basics/what-is-oidc