OAuth2 vs OpenID Connect & Azure AD Seamless SSO

OAuth 2 is an Authorization Framework.  It allows 3rd party application to get access to an HTTP Service.  It is primarily used as Token for API Access.

OpenID Connect is an Authentication Framework.  It is primarily used as ID Token for SSO.

Azure AD Seamless SSO automatically signs in users from their corporate devices to Azure Active Directory.

References.

Internet Engineering Task Force https://tools.ietf.org/html/rfc6749