One of the most important issues one needs to resolve is how to partition a URI space of the resource server application. We have two different parties trying to access it, the end users which access the resource server to get to the resources which they own and 3rd party clients which have been authorized by the end users to access some of their resources. In the former case the way the authentication is managed is completely up to the resource server application: basic authentication, two-way TLS, OpenId (more on it below), you name it.
In the latter case an OAuth filter must enforce that the 3rd party client has been registered using the provided client key and that it has a valid access token which represents the end user's approval. It's kind of the authentication and the authorization check at the same time.
Letting both parties access the resource server via the same URI(s) complicates the life for the security filters but all the parties are only aware of the single resource server URI which all of them will use.
Providing different access points to end users and clients may significantly simplify the authentication process - the possible downside is that multiple access points need to be mantained by the resource server. Both options are discussed next.