An omen card is a creature card in every zone except the stack, as well as while on the stack if not cast as an Omen. Ignore its alternative characteristics in those cases. For example, while it’s in your graveyard, Twinmaw Stormbrood is a white creature card whose mana value is 6. It can’t be the target of the reflexive ability created by Kishla Trawlers’s triggered ability (“…When you do, return target instant or sorcery card from your graveyard to your hand.”).