I have to keep thinking about it but here are my first impressions:
- Giratina-Origin has a really good typing when it comes to resistances and immunities. It gots six useful resistances (Water, Electric, Fire, Grass, Poison and Bug), and three immunities (Ground, Normal and Ghost).
- It has impressive stats, as expected from a "legendary iconic Uber": 150/100/100 for bulk, better than any OU Sweeper, and 120/120/90, great offensive pressure with average speed.
- Its movepool is even greater. You have access to Ghost and Dragon stabs, both physically and specially (pure power moves, namely Shadow Ball, Dragon Pulse, Draco Meteor, Shadow Claw, Outrage, Dragon Claw, and utilitary moves, Dragon Tail and Shadow Sneak). You have Earthquake and Earth Power, Aurasphere, Stone Edge, Iron Head (and few coverage moves may, or not, find niches: Energy Ball, Psychic, Thunderbolt...). As for support moves, Will-O-Wisp and Defog are more than enough to make him a great support overall, but we must not forget Thunder Wave, Toxic, Tailwind and Magic Coat.
Given all of these, I think this Pokemon has too many switch-ins. As in Uber, where Giratina-O can switch in Primal Groudon because of his bulk allowing to take a Stone Edge easily, Giratina-O can come on many neutral moves, like Excadrill's Iron Head. It can come on Keldeo, which is known to be a heavy hitter in OU, without investment, it can also take most of Landorus-I moves, excepted Psychic, which seems more and more unlikely (but which will come back if Giratina-O stays).
- 252 SpA Choice Specs Keldeo Hydro Pump vs. 0 HP / 8 SpD Giratina-O: 132-156 (29.9 - 35.3%) -- 27.9% chance to 3HKO
- 252 SpA Life Orb Sheer Force Landorus Psychic vs. 0 HP / 8 SpD Giratina-O: 151-178 (34.2 - 40.3%) -- guaranteed 3HKO
Meanwhile, the Griseous Orb, Giratina-O's item, is a really great item, giving Giratina-O a STAB'd Shadow Ball with 20% bonus power, allowing to hit hard most of the metagame. For example:
- 252+ SpA Griseous Orb Giratina-O Shadow Ball vs. 240 HP / 16 SpD Assault Vest Azumarill: 127-151 (31.6 - 37.6%) -- 88.9% chance to 3HKO
Moreover, the few resistances available fear both of Aurasphere and Will-O-Wisp (Bisharp, Tyranitar, Lopunny), excepted Mandibuzz, making him the only mon which totally counters Giratina-O (but still, WoW will reduce Foul Play). But what if I tell you, you can play physical Giratina-Origin?
- 252+ Atk Giratina-O Stone Edge vs. 248 HP / 136+ Def Mandibuzz: 174-206 (41.1 - 48.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
I don't know if the flaws to Giratina-Origin (Fairy, Dragon, Ice, Ghost and Dark weakness) will be enough to make him a healthy mon in OU, however I highly doubt Giratina-Origin is adapted to OU metagame as it is right now.