The Forge version of
Omniscience has some issues. In Magic you're only allowed to play a spell with one alternate cost. So if you're casting a card from your hand without paying its mana cost using
Omniscience you can't choose to do things like
Overload or Evoke at the same time. You can only use it to play it the regular way.
Currently playing an Evoke card while you have
Omniscience out lets you choose to play it normally, play it with Evoke, play it for free normally, or Evoke it for free.
I tried it with
Mizzium Mortars and
Cyclonic Rift to test
Overload and they don't seem to work with
Omniscience at all. It doesn't give you the option to cast for free.
Fist of Suns seems to work ok with
Omniscience. You can choose to cast for free or to pay wubrg.
Kicker cards are also not compatible with
Omniscience, (you don't get the option to cast for free) although since Kicker is an additional cost, not an alternate cost you should be given the option to do both. For instance play
Conqueror's Pledge for free off
Omniscience, then pay 6 to get the Kicker.
I tried
Capsize and
Whispers of the Muse to test Buyback and they also don't allow you to cast for free. Buyback is another additional cost like Kicker.
I couldn't think of any other alternate costs off the top of my head.