Steps to reproduce:

1. register a new account on enwiki
2. go to https://tools.wmflabs.org/oauth-hello-world/
3. click "Authorize this application"

Expected result: you get the OAuth dialog

Actual result: you need to log in first

This is because users are globally logged in when they log in, but not when they register. This is apparently somewhat intentional to avoid account creation spam (T18864). The OAuth central wiki will be spammed anyway due to the autocreation job that was put in place because of T74469, so there is no reason not to autologin there, and it would make the OAuth workflow smoother.

See also T94885