Did you just buy him? How long have you had him?
If he is new, Oscars tend to have a short period where they sulk and dont eat. In this case, i wouldnt worry. He'll come around eventually.
Otherwise,
What size tank do you have? What tankmates? Have you done anything different lately like adding any new fish or decor? What is your Filtration, water changes, water parameters?
All things that will help. We'll need a little more info.