I wouldn't worry too much if he does end up in Oakland- they are a young, up and coming team. Not saying they're going to become the next Pats or anything, but I expect them to be much better in another year or two. The cream usually rises to the top regardless of the situation- look at what Calvin did on the putrid Lions. His value might fluctuate slightly depending on his situation, but if he's the real deal, he'll be very good no matter where he ends up.
I would say it's going to depend on where everyone ends up if I'd take him #2 or not (if it's PPR, his value gets a nice boost obviously). If Wells and Moreno end up in very good situations, it would be hard to pass on them for the WR. If you really need a WR and don't want to risk missing out on him, take him at #2 and be happy. I traded for what became the #3 overall, and I'll gladly take whoever falls out of Wells, Moreno, or Crabtree (maybe even another RB like Brown depending on the NFL draft).