Alright here's why I think Giratina-O will be just fine in OU:
BST-wise, I cannot agree more that Giratina-O is a VERY intimidating pokemon with 150 / 100 / 100 defenses, and workable 120 / 120 offenses.
However: he's actually much more fragile than he appears at first sight.
Defensively, Giratina-O has several incredibly debilitating weaknesses (ICE / DARK / FAIRY / DRAGON / GHOST) that allow him to be 2HKO'd by several pokemon, including Bisharp, Mamoswine, Azumarill, Garchomp, Mega Gardevoir, Gengar, Mega Altaria, and Tyranitar (all without setup or choice items) which means he isn't going to be switching in and out all willy-nilly during battle. He's also OHKO'd by Latios and Kyurem-B and helplessly walled by Chansey, Clefable, and Skarmory. He's also forced to hold a Griseous Orb, which deprives him of any form of passive recovery. Not to mention, he doesn't even have a reliable recovery move, save pain-split, which is not even close to viable on a pokemon with Base 150 HP.