Feed your oscar some flake food, in addition to the other things you've been feeding him. Hole in the Head is usually caused by malnutrition. Feeding him or her only feeder fish, shrimp, crickets, and worms, is like feeding a kid junk food. He's probably lacking a lot of vegetables and veggie specific nutrients. :]
So, focus on fixing up your oscar. In his sickly state the turtle would tear him to shreds.