OK, what you're describing in addition to the fin rot is Hole-in-the-Head disease, or HITH, sometimes called other things, like lateral line erosion. This is a serious condition, usually brought about by poor water conditions, though also involving a protozoal parisite called hexamita. Oscars and quite a few other fish are susceptible.
The first and most urgent thing to do is get the water situation under control. It would help a lot if we knew what size your Oscar is and the size of the tank you're keeping him in, but the immediate action would be pretty much the same in any event. First thing is to change out about 50% of the water. Replace using water within a degree or two of the current tank water. Use de-chlorinator if you don't have aged water on hand. Just follow the instructions on the bottle. Do this every couple of days for about a week; then you can ease off on the frequency, depending on the tank size. Cut back on the amount you're feeding. No more than he'll eat in about 2-4 minutes. Decaying food is the major source of most water problems. There are some medications you can try if the symptoms are severe. Please see the thread below:
http://oscarsfishtank.com/discuss/index ... pic=2845.0I'd be surprised if the talrot doesn't clear up by itself once your water issues are taken care of, but that's fairly easily dealt with. Look in our Disease & Treatment FAQ's for the Progressive Salt Treatment. I've also used Melafix for this on occasion, but the water issue is paramount. Is this tank a new setup, or has it been running for awhile?