What you try to build would undermine the basic security that is given by the internet: that websites on one domain cannot create/change/read cookies for other domains. So no. The user needs to login at every domain seperately. OAuth or API keys only give you access from one domain to values on another domain in the name of the user. However, the user still needs to be logged in at the other domain to give you access to it from the first domain.

Edit: Seems like what I say here only applies to cookie-based authentication. Wikipedia points to LDAP for use in SSO applications (https://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol), but that is outside my horizon :P