Along the same lines, though, and a little bonus to cards like OM: the ability that makes white spells cost 1 less means that if you have a white spell with X in the cost, you get a free extra mana. so if you cast, say,
Mikaeus, the Lunarch, with OM on the board, and you have 5 mana open, you get to TAP x=4, but CAST as x=5, because OM "acts" like an extra 1.