2
it can be done with criteriaCompo
$criteria =& new CriteriaCompo(new Criteria("field_a", "a"));
$criteria_sub =& new CriteriaCompo(new Criteria("field_b", "b"));
$criteria_sub->add(new Criteria("field_c", "c"));
$criteria->add($criteria_sub, "OR");
$criteria->render();