The main confusion probably comes from the name Oauth, which seems to suggest that it is about Single Sign On/authentication, while in reality it's about granting site A access to your data at site B.
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.
Distinction which is totally useless in practice as you certainly won’t have authorization without authentication.
At that point, the separation is more about capabilities/responsibilities than "does it happen?"
(This is what I do in my day job.)
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.