A Moodle user can specify its start page within his or her user preferences. I stumbled over the fact, that logging in with a local user, the start page setting is honoured, so the successful login redirects to one of Home, Dashboard or My Courses, respectively. Depending on the user's setting.

However, logging in with OAuth2 this particular user setting is not taken into account. The successful login always redirects to document root (a.k.a. /). The user setting is simply ignored.

To replicate:

1. As the site admin, set 'defaulthomepage' ("Start page for users") to "User preference"
2. Set up an IdP, e.g. Google (because that's an easy one you can use with localhost quickly)
3. Enable auth_oauth2 plugin
4. Login via Google OIDC
5. When you're logged in, go to user preferences
6. Change the "Start page" preference to something that is not the default on a vanilla site. E.g. "My courses"
7. Log out
8. Log in again using Google OIDC
   Expected: You are taken to My courses page
   Actual: You are taken to the default / homepage (which for me was Dashboard)