Is Shaymin viable in OU? or am I better off using Celebi?
It all depends on how you want to use Shaymin. Shaymin is sometimes used over Celebi in OU because it has a powerful STAB move, Seed Flare, with an added bonus of possibly dropping the opponents SpD by -2. Celebi's offensive sets on the other hand, usually rely on the weaker Giga Drain or Leaf Storm which halves her SpA after using it. Shaymin is also not weak to Pursuit trapped, meaning it doesn't have to worry about Tyranitar or Scizor as much. All around though, Celebi is probably the best choice. She has a lot of really useful resistances and can boost her SpA to even higher levels with Nasty Plot. She has a gigantic movepool, and can even run a nasty SD set if played right. It all depends on your preferences, but Celebi is an all around better choice in OU as an offensive Grass-type.