Rule 720.3 · Omen Cards
As a player casts an omen card, the player chooses whether they cast the card normally or as an Omen.
Subrules
When casting an omen card as an Omen, only the alternative characteristics are evaluated to see if it can be cast.
While on the stack as an Omen, the spell has only its alternative characteristics.
If an Omen spell is copied, the copy is also an Omen. It has the alternative characteristics of the spell and not the normal characteristics of the card that represents the Omen spell. Any rule or effect that refers to a spell cast as an Omen refers to the copy as well.
As an Omen spell resolves, its controller shuffles it into its owner’s library instead of putting it into its owner’s graveyard as it resolves.