This is extremely common with Oscars, this is the sort of question that crops up time and time again. It is very important to give Oscars and varied diet throughout the week. Some Oscars aren't picky, other Oscars can be a complete nightmare when it comes to feeding. I have found that they older they get, the more peaky they become about food. However, occasionally you get very young Oscars that can be very fussy about food.
The best advice I can give you is to stop feeding the food that your Oscar eats at the moment completely. A healthy fish is not going to start itself to death. I do think it's a good idea to feed sparingly when the fish reaches maturity. Too often people put in far too much food for the fish. This is often when they become fussy.
So to reiterate what I have already said, stop feeding the food that the Oscar will eat. Remember that fish can go without food for a few weeks so don't feel guilty about it. Keep feeding your other Oscar as you normally do. Go to your fish store and spend some time looking the other foods that are available. If they sell frozen food, check it out, there's often quite a wide variety of food that is good for your Oscar.