If the Maracyn 2 does not cure it then it might be the less common blood fluke infection, they are best treated internally as are some stubborn bacterial infections, you could try feeding the oscar a mix of jungles antiparasite AND antibacterial medicated food for 10 days, if that doesn't cure it I don't know what will and it is still safer than many of the formalin/potassium permanganate solutions available
I was thinking it might be the fluke disease, but I will try this course first. I looked at medicated food today, but I have never fed him flakes, so I don't know if he will take them. I also have some Potassium Permanganate, I might try that if nothing else works.