I believe Giratina-O should not be OU for numerous reasons. First of all, it's typing allows it to wall most of the offensive Meta.
Giratina-O shuts down offensive threats like Keldeo, Charizard-Y, Landorus-I, Landorus-T, Thundurus-I, Venusaur (LO) and, if it's at full health,
even Mega Metagross.
Being unaffected by 3 Types, and having numerous resistances, is also a big factor to it's effectiveness. Most common types like Fire, Ground, Fighting can't do anything to it.
The best way to deal with Giratina-O is to have Latios at full HP and hope they don't crit you with Shadow Sneak.
With base defense stats of 150/100/100, it can also work as a wall and provide Defog and Will-O support.
With an amazing movepool, it is somewhat hard to predict what set it might be, unless you decide on team preview. Most popular right now, of what I've seen,
is Sub+CM. Giratina-O with a Sub up is not something to take lightly.
In my opinion, it also limits team building to an extent and forces players to resort to using Fairies or fast Dragons to beat it.
Late game, Giratina-O can sweep if dealt with threats correctly. Almost nothing that is not super-effective can OHKO this beast.
Whittling Giratina-O is no easy task too and it must be played around very carefully
Having STAB priority is also a huge plus to this things effectiveness.
Providing Will-O support, it can deal with Physical Threats such as Metagross, Bisharp, Excadrill, Landorus-T etc.
With an insane SpAtk stat, Draco Meteor is simply a must and can easily OHKO almost every non-resistant mon. Since it can also be mixed, while at the same time unpredictable, this thing should not be OU
As of now, when this reply was posted, there are limited mons that can deal with Giratina-O effectively which include fast Dragons (over base 90) and Fairies
Below are some important calcs involving Giratina-O, imo
252+ Atk Bisharp Knock Off vs. 0 HP / 0 Def Giratina-O: 228-270 (51.7 - 61.2%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Life Orb Excadrill Iron Head vs. 0 HP / 0 Def Giratina-O: 192-227 (43.5 - 51.4%) -- 99.6% chance to 2HKO after Stealth Rock
252 SpA Life Orb Gengar Shadow Ball vs. 0 HP / 0- SpD Giratina-O: 377-447 (85.4 - 101.3%) -- 6.3% chance to OHKO
180 Atk Griseous Orb Giratina-O Shadow Sneak vs. 0 HP / 24 Def Gardevoir: 194-230 (70 - 83%) -- guaranteed 2HKO after Stealth Rock
252 Atk Mega Lopunny Ice Punch vs. 0 HP / 0 Def Giratina-O: 170-202 (38.5 - 45.8%) -- guaranteed 3HKO
These are the most basic calcs as calcing how much a Hyper Voice from Mega Gardevoir would do to Giratina-O is redundant and fucking stupid