Oscar keeper here as well.
They can be real wimps when things get changed up on them. If his color is good and has no obvious signs of something going on, just give him some time like said above.
I don't remember if it was here or other sites I follow, but I read of an oscar keeper who changed a big picture on the wall directly across from his oscar's tank. The O layed on the bottom of the tank for a week or two afterwards.
