i got a 55g with a free large oscar and plec thrown in a while back, he had a real sulk on for a week and skulked at the bottom leaning on one side not eating after we moved him. he didnt eat for at least a week, and was really inactive. Within a couple of weeks though he was eating and up to all the usual oscar tricks that people describe. just give him a bit of time...
If you want to see him active, try throwing in some live river shrimp - mine went nuts for these!!!