add_action('pre_get_posts', function ($query) {
     if ('my_post_type' === $query->get('post_type')) {
          $query->set('meta_query', array(
              'relation'     => 'OR',
              'sort_empty'   => array('key' => 'order', 'compare' => 'NOT EXISTS'),
              'sort_numbers' => array('key' => 'order', 'compare' => 'EXISTS')
          ));
          $query->set('orderby', 'meta_value_num');
          $query->set('order', 'ASC');
     }
 });