I agree with Mush... he needs a much larger tank. You said you've had him since he was small... how long have you had him? How big is he now?
Without water params to help, there's not much anyone is going to be able to tell you other than to get this fish into at least 55 - 75 gallons. Keep in mind that if you go 55, eventually you'll still need 75 gallons, as a 55 is too narrow to allow a full grown oscar to turn around properly.
What kind of foods are you feeding him? How often? How long has he been like this? The locked jaw could be caused from a few different things, illness or injury. I will look into any possible help you could give him, but IF there is something that can be done, I know its not easy. How long has it been since the fish ate something?