There are several ways to counter the omoplata:
1. Roll forward. This of course then puts you under side control or even under mount. Should be used as a last resort.
2. Control his other ankle so he can't triangle his legs, get posture and pull your arm free.
3. Put your head on the floor and jump to the other side, cross face him and get side control. This can be countered though by him sitting up as you jump over.
4. If he has your left arm, your left hand grabs your belt, your right hand reaches under and grabs his belt, fall on your right side and roll underneath as you pull him over you. He'll land on his knees and you should be able to pull your arm free.
5. Put the same side knee on his belly, posture up hard and then fall back into kneebar.
Number 2 is the best method, because you're not letting him have anything.