A massive dead robot with Omega Supreme's head and Ratchet's body can be seen from the Decepticon spawn on Desolation. I think you find a bunch of his heads (on a smaller scale) on Sewers, though I'm not certain. Any other maps his head is on? I could have sworn it's very prominent in the game, for some reason. I think he's also on a few Escalation maps as well, like Ancients.