With 0.12, writefreely supports OAuth login . However, this requires the server to have open registrations enabled (otherwise logging in via OAuth returns ‘This user does not exist’ error). While in theory this makes sense, it results in requiring the instance to accept email-based sign up as well as OAuth without being able to distinguish between the two.
I suggest changing the ‘Open Registration’ to ‘Open Local Registration’ and only allowing the ‘Sign Up’ link to show if it is set. If one of the OAuth log in methods are configured, create accounts for successful login without checking the ‘Open Local Registration’ configuration option.