There could be a number of reasons for this problem to occur and probrably the one thing you can do to have the quickest impact without spending money is to do a water change. Often times this is a water quality issue so check your water parameters. It may also help to let us know what those numbers are as well in case they can give insight into the possible cause. I would think that with keeping such a messy fish like an Oscar, water quality can change pretty rapidly so be aware of that.
What is the size of the tank you have?
What other fish are in with this one, if any?
Is there any apparent wounds near the eye that could signal an injury from fighting as the cause?
The other reasons for this type of thing to occur is quite broad and range from bacterial to parasite infectations. Usually, when you cannot be sure exactly what is to blame here, a broad spectrum antibiotic would be suggested. I would first look at water quality and then if you can be confident water is not the issue, you could go with a med like the Maracyn product. I would use Maracyn "One" and not Maracyn Two to start off with personally. From what I understand, the liklihood of Maracyn One killing off your benifical bacteria is more slim than with maracyn two. There is really no guarantee it will work though and it is still a shot in the dark, but as some people might say, sometimes it is ok to shoot first and ask questions later.
Also, I dont think that you can use the injured or abnormal eye to guage how well the treatment is going. According to
http://www.fish-disease.net/diseases.htm, the eye may stay the way it is now which will not hurt the fish so you goal may be more along the lines of stopping the problem from progressing rather than reversing it completely.
