Difficult to say what is wrong since your other Oscar is behaving normally and you say your water conditions are okay. Are you sure that the little Oscar is not getting bullied?
Have you tested for nitrate? probably a good idea if you haven't. Also, check the oscars breathing, is it laboured, or normal? rapid gill movement and opening and closing of the mouth constantly could be a sign of problems. Unfortunately I can't really help you any further at the moment as the information you have given us isn't a cause for concern. If the water was in poor condition, then we would have something to go on. The best thing you can do it as keep an eye on the Oscar, if this carries on.