Don't treat until you know what is wrong or you may create more problems. Are the sores growing? Have you sprayed any chemicals in the house lately? Burned scented candles? Could kids have put something into the tank? What you describe sounds a lot like poisoning, but it's hard to know.
You said you checked your water and it was fine. Fine is not a number. Ammonia needs to be zero, nitrite needs to be zero, and nitrates should be below ten for Oscars. Anything else you can tell us? Could other fish be biting him or striking his operculum?