There are actually a lot of sweeps from the omoplata position. Which one are you referring to? Here are some of the ones that I know about:
1) From omoplata position, kick your leg (the one under their armpit) forward forcing the guy to roll forward. You sit up and *poof* you're in side control. You must control the guy as soon as his back hits the mat or he'll keep rolling up to his knees.
2) From omoplata position, he's trying to stack you, put his knee on your chest, or knee on your face. You roll backwards grabbing a hold of his closest leg and yanking upwards as you're rolling. You wind up sweeping him into *poof* side control.
3) From omoplata position, he's maintaining good posture so it's hard to do sweep 1. Grab the leg closest to you (actually hugging it to your chest). Then extend your hips (stetching him out). Once he's stretched out you roll him sideways into *poof* side control. There are many variations of this sweep. If his weight is ever off-center and leaning in the direction you want to roll him, you can pull this off without stretching him out.
4) From omoplata position, he's maintaining a good base. You transition to a pendulum sweep (grabbing the closest leg, keeping his upper body down and swinging the free leg instead of triangling with it).
There's more that I'm forgetting to mention here. Let us know what sweep you need help with and I'm sure you'll get more helpful replies.