If you cast an omen card as an Omen, use only its alternative characteristics to determine whether it’s legal to cast that spell. For example, if you control Thundermane Dragon (“You may cast creature spells with power 4 or greater from the top of your library.”) and Twinmaw Stormbrood is on top of your library, you can cast Twinmaw Stormbrood, but not Charring Bite.