Was my comment unclear? "Oauth is NOT single sign-on" single sign-on = authentication. "sharing data across services" = authorizing your data from service A to be used in service B.

> the distinction between authentication and authorization

Distinction which is totally useless in practice as you certainly won’t have authorization without authentication.

The distinction is irrelevant if you just consider that authorization (authZ) always happens after authentication (authN) but it is important when you realize that different components/systems/protocols might be used in each.

At that point, the separation is more about capabilities/responsibilities than "does it happen?"

(This is what I do in my day job.)

Somewhat. One key “feature” of OAuth is that it completely omits the authentication process.