Ogresmasher doesn’t need bonuses versus ogres, or confer beneficial properties. It is simply an oversized sledgehammer with such terrible crushing power that it splatters anything it’s swung at. On top of modest damage bonuses, it has chances to instakill sufficiently small monsters, and may inflict negative status effects on large monsters like stunning or slowness (or bleeding, if that is implemented). Probably better if war hammers are made into two-handed weapons.