Oscars can be for lack of a better description, Drama Queens lol. Mine have done the same thing before, it is not unusual for them to just rest on the bottom of the tank and mope about. They usually do it when they've eaten too much or sometimes when they are in a new environment, it is normal behavior for them and after a little bit they will come around. Also something to note, if they do over eat they tend to swim a little sideways until they digest their food or throw up what they cant finish.