Been done and done again. Rewritten version:
-If God is omnipotent, then he can create a rock so heavy even He could not lift it. [O => (R & ~L)]
-If He can not create said rock, or can lift the rock, then He is not omnipotent. [(~R + L) => ~O]
-If He can create the rock, then He can not lift the rock; therefore, God is not omnipotent. (R => ~L /.`. ~O)
It's supposed to prove that God isn't all-powerful. Not exactly an idea that busts the brain.