Hearthpwn

Mulligan:

Mulligan for Eviscerate, Deadly Poison and Coldlight Oracle with Shadowstep.

Playstyle:

Try to clear his board with removal and return effects.  You will often only use your hero power for the first couple turns.  Don't play Coldlight Oracle or Antique Healbot unless you have a way to return them or if you have Gang Up.  Sylvanas Windrunner is good to play when Omokk only has one big threat because it basically acts as a Mind Control.  Try to use one Gang Up on Coldlight Oracle and the other one on Antique Healbot.  Be cautious when using Sap on Dunemaul Shaman because he could replay it and give it Charge to deal 14 damage to you.  The deck plays pretty much like a normal mill rogue.

You could add Deathlord and/or Dancing Swords to thin out Omokk's deck more, but I don't really like them that much since you're essentially paying mana to give your opponent something.  Also I didn't add Preparation because I still don't have that card, but it combos well with Vanish and all the other spells.