Hearthpwn

Found this easy solution for beating Highlord Omokk on heroic and successfully won another 4 times to  test it out. This is basically a modified version of 'Echo Giants Mage' with a minimum of minions to cancel out his hero power. I won most of the games on fatigue, the basic "freeze mage" mechanic works perfectly to counter his board.

It is too hard to get board control against him when he can kill one of your minions every turn for free so focus on just stalling his board and clearing it out until you can get your giants in play. Mulligan for either Mad Scientist for your secrets or Neurbian Egg to get a small minion on the board. 

Try and use your first duplicate for Sylvanas, you'll be able to steal each of the Ogre Ninjas he puts down for the next two turns, buying you more time. If you don't have Sylvanas, substitute in a Loot Hoarder for the card draw, or possibly to draw fire away from your other minions.