Though it could be from fighting is sounds like Hole In The Head, or as it is often shortened on the forums HITH, you should take a picture and post just to make sure, but it is fairly common in Oscars seemingly.
What size of tank? filtration? tankmates does your Oscar have?
Your local LFS will sell many types of medication for treating but before applying any chems make sure it is HITH first, the best medicine is prevention, step up your water change routine, lighten your stock and/or upgrade your tank.