I believe you can check this setting in discourse auth_overrides_email for this.
And you can use this sight setting for this: auth_skip_create_confirm
When signing up via external auth, skip the create account popup. Best used alongside auth_overrides_email, auth_overrides_username and auth_overrides_name.
There is also the auth_immediately setting which might do what you are after as well:
Automatically redirect to the external login system without user interaction. This only takes effect when login_required is true, and there is only one external authentication method
You can visit /u/admin-login to bypass oauth.