Giratina-O can check/counter/1v1:
Keldeo, Lando-I, Mega Metagross, Heatran, Lando-T, Mega Lopunny, Mega Scizor, Talonflame, Thundurus, Celebi, Mega Charizard-Y, Excadrill, Hippowdon, Manaphy, Mega Manectric, Rotom-W, (Mega) Slowbro, Starmie, Tornadus-T, Mega Gallade, Gyarados, Jirachi, Politoed, Volcarona. And almost everything below A rank that does not have a SE move to scare it out.
With 3 amazing immunities, 150/100/100 bulk it can pivot and check so many threats while maintaining good offensive presence with 120/120 mixed offenses. It also has great utility with defog/d-tail and has a great speed tier for a pivot. I do not see why this pokemon would not be used on just about every type of playstyle.
It can spinblock for HO, pivot for offense(checks so many threats that offense hates), defog for balance, defog and phaze for stall. With so many possible roles I think Giratina-O would be top-tier in OU
The balance it brings to each team would be welcome however as it is not overwhelming on the offensive side nor is it unbreakable with many offensive threats such as, M-Alt, Gengar, Diancie, Clefable, Bisharp, Mega Alakazam, Bulk Chomp, Lati@s, Gardevoir, TTar and more being quite common place on many of todays teams. Of course it would take a bit of time for the meta to settle but I don't think Giratina-O would be an unstoppable force in the OU metagame.
Lack of proper items hinder it a lot as well.
However I believe the problem lies in where it would be used on just about every team and it would be overcentralizing seeing as a fairy type will be almost mandatory on each team.