OAuth has the following four main actors:
Resource Owner (RO): the user
Client: the web or mobile app
Authorization Service (AS): OAuth 2.0 server
Resource Server (RS): where the actual service is stored
With oAuth the AS only provides the client with a security token; it provides no access to the users identity.
An identity layer on top OAuth which makes OAauth identity aware.
References:
How To Control User Identity Within Microservices
3 Unique Authorization Applications of OpenID Connect