So I was playing Zilean because quite frankly I find it hilarious, when half-way through my game (which I was winning) the enemy Riven states that everyone on our team was OP. Including me there was a: Zilean, Graves, Jarvan, Morgana, and Master Yi (re-re).
Now, maybe, MAYBE you can say Morg is OP because of her shield, but I quite frankly don't buy it. Jarv is a tank and has some CC, but he's just a tank really. I guess he can build AD, but I don't see the OP part. Graves was carrying, as I was supporting him bot. Master Yi, well he can hard carry but OP? Build armor and he's done.
So I decided to say, "I'm not OP. :S" and Riven responds, "YES YOU ARE MR REVIVE EVERY (censored) CHAMP THAT DIES!"
I mean, my ult IS pretty hilarious, but I don't think it's OP, I mean, it might help in team fights and it can save someone in a bind when partnered with someone, but really, I don't think there is ANY reason to call Zilean OP.
Personally, I think Riven was just raging, but Zilean being OP is a funny concept to think about.