I've had many Oscars and one thing you can always count on is they are always bumping,raming, or subbing on something. Sometimes out of frustration, sometimes out of aggression, and sometimes because they just get bored. As they do these things sometimes they just get nicked up like kids playing in a school yard. Keep an eye on it. If its just the one sore it will probably heal on its own. If you see more sores of the same type, then there is something else going on. Your Oscar looks otherwise healthy, so I would guess its nothing serious.