Omega Mark XII isn't an official mark; just an optional boss.

I believe that you have to accept the Yiazmat hunt first before you can fight him. If you try to go to him otherwise, then he will just be immobile/asleep when you get there, and you won't be able to fight him.

As BG-57 stated, you need 10 Espers before you can get the path to Zodiark open. Also, you have to complete the Mindflayer hunt.