og_bitch.php

<?php 
/**
 * Loads a group ID from a node ID
 */
function artroom_ui_og_load_from_nid($nid) {
  static $groups;
  
  if (empty($groups)) {
    $q = db_select('og');
    $q->fields('og', array('gid', 'etid'));
    $q->condition('entity_type', 'node', '=');
    foreach ($q->execute()->fetchAll() as $row) {
      $groups[$row->etid] = $row->gid;
    }
  }
  
  return og_load($groups[$nid]);
}