http://www.jamiesutherland.com/2011/07/01/doctrine-2-odm-querybuilder-addor/
Here is how it’s done
$result = $this->_dm->createQueryBuilder('HOD\Model\Person');
$result->addOr($result->expr()->field('firstName')->equals(new \MongoRegex("/^{$query}/i")))
->addOr($result->expr()->field('lastName')->equals(new \MongoRegex("/^{$query}/i")));
return $result->getQuery()->execute();