It's normal for a sulking oscar getting used to his environment. Keeping the light off, lighting the tank with only room light will help. Limiting commotion around the tank will help as well. Oscars are known to behave this way when their environment is disturbed. Mine does.
Get your water tested at your lfs, ask them to write down the numbers so you can post them here. Make sure you have a larger tank in the works, oscars get big fast, and need lots of filtration as well.
My oscar started out as a 1" fish a little over a year ago. He went from a 10 gallon to a 20, a 30, then a 72. He's a good 8" right now. He pouted for days with every tank upgrade, but he got over it.