Does not look like any hole in the head disease i have ever seen. Oscars are pretty rowdy and i think this one has found away to cut him self on some thing or maybe a burn from the heater. I would maybe add some salt to the tank and just keep and eye on it. The salt will help keep bacteria or parasites that may be in the water from attacking the spot. And the salt will also help to calm the big guy since he might be stressed from the wound.
