The best chance for the oscar to heal is if you keep the water quality up. Buy test kits for ammonia, nitrite, and nitrate, and make sure that the water is within the safe levels as instructed by the test kit.
Also keep very close watch over the fish; if the scratched area grows a white fuzz, you want to treat immediately with an anti-fungal medication.
By the way, you shouldn't ever do a full water change. It stresses fish, and there's really no disease whose medications tell you in the instructions to do that. Never remove the fish from the aquarium. You don't have to. Simply gravel siphon the aquarium that the fish is in to remove water.
And by the way I hope the oscar is in a really huge tank because they grow to be a foot long. If it's smaller than 70 gallons, then you might want to consider getting a bigger tank.
