I am having problems with my Oscar help me Please?

I have a 55 gallon tank I just started, it has an oscar, jack dempsey, and a biker eel. I recently added two plecos to the tank so it would stay cleaner and and now my oscar swims around with his mouth wide open and gills flared out and will not eat or close his mouth. Intel the plecos went in all of the fish where happy and ate well the oscar more than any of the others and I dont know what to do anyone have any ideas what is wrong?