Yeah, a pity that the build in Oauth2 seems to not do SSO. It seems to work, just not provide any authentification services, which strictly speaking is probably the correct implementation of Oauth2 as the standard isn’t meant for it (but rather the “add-on” to it OpenID Connect). However there is also the “old” Oauth2 plug-in that might be a better starting point to implement a SSO for Nextcloud? https://github.com/soudis/oauth2
You saw this thread? Anyone got DokuWiki integrated with Nextcloud?
I have not really gotten around getting SAML2 to work, but at the end I linked to my repository that allows SimpleSAMLphp to authenticate against a Nextcloud12 database and pull the parameters such as groups etc. Totally untested, but maybe a starting point.
However I am starting to think LDAP might be after all the currently best solution (but my shared host OVH doesn’t have php-ldap enabled 
). I am just not so sure how advisable it is to use it in the public, i.e. not in an firewalled intranet as it seems to have been intended. Any advise on that?
However I came across these Nextcloud plugin that would allow administrating LDAP users from within Nextcloud:
Another alternative might be this:
Which allows among many other backends to authenticate against an IMAP email server and thus could allow easy integration with some groupware?