well oscars do sulk when put in a new tank- sometimes it can last for a few days lol.. make sure you have some plants (fake are better than live with oscars) in there-a floating long one would be cool, and some rock work like you said is great. i would put some driftwood in there-a few large chunks..
is the tank newly setup? if so, check your water parameters and make sure you have no ammonia, no nitrItes, and that your NitrAtes are 40 ppm or less- (i keep mine below 40-some do, some dont lol)