Some EntityFieldQuery's for working with OG subgroups.

examples.php

<?php

/**
 * Gets all content contained in a group and all it's sub-groups.
 *
 * @param integer $gid
 *   The OG group gid (the nid of the node).
 *
 * @return array
 *   An array of node ids.
 */
function example_get_all_content($gid) {
  $content = array();

  $query = new EntityFieldQuery();
  $query
    ->entityCondition('entity_type', 'og_membership')
    ->propertyCondition('entity_type', 'node', '=')
    ->propertyCondition('group_type', 'node', '=')
    ->propertyCondition('gid', $gid, '=')
    ->propertyCondition('state', OG_STATE_ACTIVE, '=');
  $result = $query->execute();

  if (!empty($result['og_membership'])) {
    $memberships = og_membership_load_multiple(array_keys($result['og_membership']));
    foreach ($memberships as $membership) {
      $content[] = $membership->etid;

      if ($membership->field_name == 'oa_parent_space') {
        $content = array_merge($content, example_get_all_content($membership->etid));
      }
    }
  }

  return $content;
}

/*
 * Here is a random other one..
 */

    // Find all the user's group memberships.
    $query = new EntityFieldQuery();
    $query
      ->entityCondition('entity_type', 'og_membership')
      ->propertyCondition('entity_type', 'user', '=')
      ->propertyCondition('etid', $account->uid, '=')
      ->propertyCondition('group_type', 'node', '=')
      ->propertyCondition('state', OG_STATE_ACTIVE, '=');
    $result = $query->execute();