Users that cannot login with OAuth

What do you mean you changed to oauth?  Oauth is a supplement to the community login process, its not meant to stand alone.   The link is only created when a user successfully authenticates with an oauth client for the first time, or logins in with another and successfully provides their password.

If a user knows their password then they should be able to link the original account to an oauth client assuming its the same username/email.  This is of course unless the process or UI has been altered in some way.    If they don't know their password it will have to be reset.

Hi Patrick, What I mean is that we started to use an oauth client to login into the community.So people who started to login using our oauth client, had a link created for them, but the rest who used to login using the telligent login have no link entry in the database that to link them to a oauth client.They are now trying to login, and the login process fails, how can we help all the remaining people get on board?

This sounds like a problem in your client somewhere.  Your client should be getting information from whatever oauth service you are using and try to reconcile an exiting account, which should prompt the flow to ask for the community password to create the link.