I had read on another thread that putting the sick fish in a small container with a slice of FRESH Ginger may help if it is bacterial but I would research it more before i tried it myself
From what I know, pop-eye is a symptom, not a disease. And unfortunately I can't give any ready advice for treating what may be causing it, although internal bacterial infection would be at the top of my list. If Maracyn Plus can't help you, I don't know what else to suggest other than move the oscar to a hospital tank, because it will be cheaper to treat.
I've never personally had to deal with pop-eye, and I am sorry that you have to because O's are great fish, but I would prepare for the worst and hope for the best.
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