As usual - it depends :)
In our case, we have a need to share roles across a number of endpoints / APIs / applications in a single-sign-on environment. These roles are managed centrally through OAuth tokens. What the roles /mean/ when a specific application or endpoint receives them is defined locally (as I described in the example above).
If you have a single application, and it already has local permissions / role management capability, then you have little need to move that elsewhere. Indeed your driver for using OAuth is likely different too, typically such applications need to accept identity assertions from other environments such as Google or Facebook, whereas we are building an SSO platform for ourselves... YMMV!
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