How to force openid login from a single provider in Drupal 7?
You must create a custom module to generate a page "/login" that start OpenID authenticate.

Create a module with this code :

/**
 * Implements hook_menu().
 */
function mymodule_menu() {

  $items['login'] = array(
    'title' =&gt; 'OpenID Login',
    'page callback' =&gt; 'openid_login',
    'access callback' =&gt; 'user_is_anonymous',
    'type' =&gt; MENU_CALLBACK,
  );

  return $items;
}

/*
 * Menu callback; start openid authenticate
 */
function mymodule_login() {

  $return_to = url('openid/authenticate', array('absolute' =&gt; TRUE, 'query' =&gt; array('destination' =&gt; 'user')));

  openid_begin("https://www.myopenid.com/", $return_to, array());
}


The example works with the myopenid.com provider.
Go to "/login" on your website to log in.