Is OpenID Connect the one where it issues a JWT that you can then verify and exchange for a session cookie? If so, I love the API. Very easy to use, and easy to integrate multiple sources.
The first and most important coupling point is the ID Token. There are three types of tokens issued by Open ID Connect: Access Tokens and Refresh tokens are opaque, just as in OAuth 2. ID Tokens, on the other hand, most follow a more-or-less well-defined JWT format (as much as JWT can be well-defined - it is a somewhat wild west of a standard).
The rest of the places you could use JWT are in the spec, but I admit I've never seen them in the wild (and I would probably avoid them like the plague if I was trying to implement a _secure_ subset of an OpenID Connect IdP):
1. JWT request signing (and even encryption!) 2. JWT for signing and encrypting any JSON response 3. JWT for client authentication 4. JWT for aggregated claims
And I may have missed more things.
If you want to implement Open ID Connect without being crazy, forget about all of these and only use/implement the Auth Code flow with a client secret. You should still verify the ID Token according to the spec, but if you fail to verify it's no longer an issue, since this is a value you get directly from the the Authorization Server on an authenticated TLS channel (the same channel you'd trust for getting the ID Token signing keys).
Either way, pretty much everything in OpenID Connect is optional and/or implementation specific. OIDC providers have to be integrated one by one, adjusting to their slight differences.
I've got to say, I've seen providers without proper discovery, I've seen providers with strange hybrid flows and I'm pretty sure most providers don't implement the request/response signing and encryption (after all, if you want insanity, you'll be more than happy with SAML). But I've never seen a provider who doesn't implement an ID token.
I think everybody would agree that OpenID Connect without an ID token is just plain OAuth 2.
I've had providers with the id_token and access_token being the same value. Other where they were different values. One that didn't support refresh_token at all.
Then a variety of screw ups where the required claims in the JWT tokens are missing or incorrect.
It's wild really. It always takes a bit of work to integrate any OpenID Connect provider. Most libraries only care to handle Google or Facebook login.
From example, the request object in OpenID Connect (that can prevent authorization request parameters from tampering) is only just now coming out as an RFC https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-19