There are several forum conversations regarding the fact that there is no option to have Single Logout for Oauth2. 

This is a security issue for shared computers where students log out of Moodle but are not logged out of the IDP and as such the next student would come along and be logged in as the previous student.

This really needs to be an option in the Oauth settings.

As an admin, I should be able to select if Logging out of Moodle also logs out of IDP.

As a user, if the option for Single Logout is in force, logging out of Moodle should redirect me to the Idp logout page so I know that I am logging out of everything that is connected through that service.