A little melafix in the water does wonders for beat up fish. I dont recommend medicating, but I use melafix like hot sauce- I put that on everything! Its the neosporin of the aquarium world. Clean warm water is your A-1 priority, if you have some melafix around, add it.
Did you just get the oscar home? If you have a new fish, stressed from the pet store expeiernce, in new surroundings and you have a bully in a tank who's already established his territory chances are the new guy will get beat up regardless of size or temperment. When your oscar is properly quarentined, is active and eating well, and ready to be introduced, you can either redecorate the tank by moving stuff around and confusing the bully a bit, put in a tank divider so the oscar gets comfortable in his new home without being terrorized, or your best bet IME - switch tanks for a couple days- . Let the oscar get a feel for his new digs without any harassment and confuse the bully by switching him out to a new tank, when you put him back he'll be a little out of sorts and wont be so willing to make trouble. They will be on more even ground when he returns.