honestly defensive omastar is a lot better than corsola. The only things corsola really has going for it over Omastar are Regenerator (which leaves it without Natural Cure) and Recover...but when your defenses are 55/85/85 it really doesn't matter. Omastar's 70/125/70, ability to lay all hazards, and special attack strong enough to actually hurt stuff make it a much better bet. Since it actually checks stuff and is useful aside from setting rocks...corsola just isn't.
but tbh neither is really good playing a defensive role. Water/Rock is really bad defensive typing in RU right now and Corsola gets beat by all the common spinners, especially with Claydol gone now.