Extends Drupal module OpenId Connect to block new account creation if registration is restricted to administrators in Account settings. Requires patch: https://www.drupal.org/node/2559543#comment-10424525

openid_connect_restrict_user_register.info

name = OpenID Connect Restrict User Register
description = Uses the OpenID Connect API to block new account creation if registration is restricted to administrators. Requires #2559543 patch.
core = 7.x
version = 7.x-1.0

openid_connect_restrict_user_register.module

<?php
/**
 * @file
 * Code for the OpenID Connect Restrict User Register module.
 */

function openid_connect_restrict_user_register_openid_connect_pre_login($tokens, $account, $userinfo, $client_name) {
  // Don't allow new account creation if registration is restricted to
  // administrators.
  if (variable_get('user_register', USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL) == USER_REGISTER_ADMINISTRATORS_ONLY) {
    drupal_set_message(t('Only administrators can register new accounts. If you are registered with this site, you may need to connect your account to log in with an external provider.'));

    return FALSE;
  }
  return TRUE;
}