Not OP, but he's definitely strong in situations where the opposing team doesn't believe in playing tanky champions or building defense. He is also really, really good at punishing people that don't look at the minimap.
As soon as he hits 6, it's pretty much a free kill/ace against players like that.
Despite this, Wu is balanced in the sense that he has to build damage to do damage, so if he's doing tons of damage he will die to focus.
Oracles on a tanky champ with CC, building some defense/health, and not playing a protect-the-carry team are the best ways to counter him.
You can't ever (in any circumstance) go glass cannon against him.
He will wreck you every time.