I can understand the implications of this, but I really think you guys missed the mark on this one. I'm happy to advocate the removal of the 'cover legends are broken, end-of' stigma beyond just Kyurem Black, but good lord you picked a Hell of a mon to try with because this thing is just borked. Providing easy offensive and defensive coverage backed up by a vast offensive and supporting movepool make it excel beyond most others at any role it tries to pull, and with the ridiculous things it blanket checks or just flat-out hard-walls it will have you forcing switches left and right, it'll make hazard stack styles a piss-easy way to wear opposing teams down, especially with access to Roar for phasing and Shadow Force in conjunction with Toxic Spikes does not make things easier; Roar Will-O Phantom Force Gira with Rocks Toxic Spikes support will be team-grinding 101 and it honestly sounds more skilless than the notorious Baton Pass.
Access to Rest-Talk isn't exactly the shit recovery it's made out to be, it makes more defensive utility sets that much more hard to take down, given that it deals with a nice chunk of current potent wallbreakers anyway, Rest helps it bounce back from status and is pretty damn good for the less offensively oriented sets, though those sets value their great coverage anyway so it's not a major loss, though if you really want an offensive Giratina set with recovery then CroTina is an option.
Speaking of the Cro set, or any set really but Cro pretty much needs to follow this, this thing's synergy with the already overabused KeldeGross core is pretty fucking solid, KeldeGross can handle either type immune to Gira's STAB and very much threaten it's counters (lol) and in turn Gira can shut down the powerful beasts that threatened the original KeldeGross like Zard Y and Lando-I; KeldeGross was potent enough as it was and was very much a likely incentive prompting this suspect, but instead of encouraging creativity by using new cores with mons that can handle KeldeGross better, the KeldeGrossTina core was born and will likely be on half of the teams around, their synergy and coverage is just busted.
Counters.
180 Atk Griseous Orb Giratina-O Shadow Force vs. 252 HP / 160 Def Clefable: 223-264 (56.5 - 67%) -- guaranteed 2HKO after Leftovers recovery
180 Atk Griseous Orb Giratina-O Shadow Force vs. 252 HP / 252+ Def Clefable: 186-219 (47.2 - 55.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252+ SpA Giratina-O Earth Power vs. 248 HP / 192+ SpD Heatran: 296-352 (76.8 - 91.4%) -- guaranteed 2HKO after Leftovers recovery
180 Atk Griseous Orb Giratina-O Shadow Force vs. 0 HP / 24 Def Mega Gardevoir: 576-680 (207.9 - 245.4%) -- guaranteed OHKO
180 Atk Griseous Orb Giratina-O Shadow Force vs. 252 HP / 252+ Def Klefki: 160-190 (50.3 - 59.7%) -- 85.9% chance to 2HKO after Leftovers recovery
252+ SpA Griseous Orb Giratina-O Draco Meteor vs. 56 HP / 0 SpD Kyurem-B: 576-680 (142.2 - 167.9%) -- guaranteed OHKO
180 Atk Griseous Orb Giratina-O Shadow Force vs. 248 HP / 0+ Def Mega Scizor: 144-169 (41.9 - 49.2%) -- 83.2% chance to 2HKO after Stealth Rock
180 Atk Griseous Orb Giratina-O Shadow Force vs. 252 HP / 252+ Def Mega Altaria: 142-168 (40.1 - 47.4%) -- 48.4% chance to 2HKO after Stealth Rock
252+ SpA Giratina-O Aura Sphere vs. 4 HP / 0 SpD Assault Vest Bisharp: 324-384 (119.1 - 141.1%) -- guaranteed OHKO
252+ SpA Giratina-O Aura Sphere vs. 248 HP / 252+ SpD Tyranitar in Sand: 176-208 (43.6 - 51.6%) -- 56.3% chance to 2HKO after Stealth Rock and Leftovers recovery
252+ SpA Griseous Orb Giratina-O Draco Meteor vs. 252 HP / 0 SpD Mega Venusaur: 226-267 (62 - 73.3%) -- guaranteed 2HKO
What counters? This is just against the mixed attacking variant with varying coverage facing off against various Pokemon I've seen people list in this thread as 'counters', the only two mons I've seen actually describable as counters to this are Ferrothorn (though I'm not sure how common HP Fire is or will be) and Audino. Audino! Mega fucking Audino in OU! Is this the creativity people want in OU? 50% of teams running KeldeGrossTina offence based balance and the other 50% running GirAudinoThorn hazard stack balance?
Speaking of which...
Giratina-O's 150/100/100 defenses will help balanced teams deal with threats such as Charizard-Y, Landorus-I and Serperior.
This was the anvil that broke the camel's back for me. Forgive me if I'm misinterpreting this, but Giratina is coming home because all these offensive powerhouses are giving balance a tough time. Not like Balance isn't already regarded as the best playstyle anyway with how much less it suffers from match-up issues compared to other playstyles; yes, it's not flawless, it can be greatly threatened by some things, that's how it goes. It still has an advantage simply on the basis of being balanced and introducing a massive blanket check to make it an even better playstyle is not what this meta needs. Yeah, Gira holds utility on stall and hyper offence too, but the support it brings to balance, a playstyle which arguably really doesn't need it, is too much. I have no sympathy with balance players having to deal with powerhouse wallbreakers than I do with fellow offensive players getting screwed by 'match-up issues'.
Overall, this seems like a two wrongs make a right scenario, where the answer to handling possibly borked mons in the meta is to introduce another borked mon. It just won't work out. Unless you arranged this suspect for the sake of promoting creativity with Mega Audino based hazard stack balance, in which case April Fools to you too. I've not had much time to play around in this meta, but from what I've played in it I've not hated OU this much since Mega Mence. Perhaps it's a case of Mega Slowbro or Shiny-Eyeballs syndrome where it just seems out of whack because it's new here and we're not quite sure what to do to deal with it, but eventually we will find solutions without having to overcentralise the meta too much. I very much doubt it though.