What are your tank parameters? Temp, pH, Ammonia, nitrIte and nitrAte?
How long have you had the fish?
How long has your tank been set up?
Do you have any decor in the tank?
What size tank and approximately how large is your Oscar?
Answering those questions will help tremendously.
Your fish most definitely needs treatment but the best course will depend on the specifics of the information provided above. If you could also describe how his behavior is more specifically than acting ill that may also be beneficial. Is he eating, laying on the ground, floating at the surface, swimming strangely, etc. The more info you provide the better answer you will receive and the more likely we can help you save your fish.
Ive kept Oscars for close to 25 years. Ive NEVER seen that before. Overall he looks in poor health though as his dorsal fin looks very thin and ragged, and his pectoral fin appears to be ragged as well. Poor guy is in rough shape.
It looks like True body fungus. its usually in a lot of aquariums but doesn't turn into a disease unless the fish is injured or under extreme stress. Sadly it can take less than a week to become fatal if not treated. Treatment is limited either with anti bacterial drugs bought at pet stores and used exactly as instructed or with salt and that can be harmful to fresh water fish and environments. I have no personal experience in treatment. Sorry about your oscar, I hope he makes it.