Hi Kevin,
In the meantime we are still using LDAP for agent & end users to log in other than the two accounts we are using to try and get over to oauth. Thankfully agents can still log in with it and users 99% of the time use email anyway. If needed I can go through process in that link to get them access.
I have set the named user in "ost_user_account" to null in backend field and retested, same result. I have also changed what I believe is our second user to null (compared user_id found in another table) and same result. Its hard to be 100% sure as only 1 user in ost_user_account has a username field that isn't null.