Hi David,
I a not sure how exactly OAuth2 works, but anyway, in Kentico 7 the only easy way to do external authentication is this:
https://kentico.atlassian.net/wiki/display/K7SG/Managing+external+authentication
Here is the same for Kentico 8:
https://docs.kentico.com/display/K8/Handling+custom+external+authentication
In case this would not be enough, you may want to consider using claim-based authentication as described here:
https://docs.kentico.com/display/K8/Claims-based+authentication
Here is a full list of authentication possibilities for Kentico 8:
https://docs.kentico.com/display/K8/Managing+external+authentication
Let me know if you need any more guidance