> OpenID is a single sign-on protocol built on top of Oauth.

Note that there's older OpenID (without "Connect"), which, to my knowledge, is more or less dead nowadays, and OpenID Connect, which is indeed built on top of OAuth. Both are indeed SSO, though.

> That’s what Oauth is though.

Splitting hairs but no, Oauth has nothing to do with authentication. An introductory article like this should address the distinction between authentication and authorization in the first section IMO.

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.