More than likely this is an injury that has gotten infected. You could treat with a bacterial type medicene to help it heal, or you could just increase your water changes and hope that it will naturally heal. Unfortunately eyes seem to me the only thing fish can not grow back. Once the eye is damaged it may remain cloudy, or white after the infection has been dealt with, and it may become blind in this one eye.
If you decide to treat medically I would highly suggest moving this fish to a hospital tank for the treatment. You do not want to treat your established tank, or your fish that are not sick.
That is about all I can add, since I don't know how big these oscars are or how big your tank is. But I will say that oscars are very dirty fish, and water quality becomes an important issue when keeping large fish, that eat a lot, in an aquarium. Make sure you are doing substantial water changes on frequent basis, regardless of how big your filter is. They will love you for it!