Some calcs for thoughts.
252 Atk Tough Claws Mega Metagross Ice Punch vs. 248 HP / 248+ Def Giratina-O: 170-202 (33.7 - 40.1%) -- guaranteed 3HKO
252 Atk Mega Lopunny High Jump Kick vs. 248 HP / 248+ Def Giratina-O: 159-187 (31.6 - 37.1%) -- 83.8% chance to 3HKO
252 SpA Life Orb Landorus Hidden Power Ice vs. 248 HP / 8 SpD Giratina-O: 156-185 (31 - 36.7%) -- 65.2% chance to 3HKO
252 SpA Mega Manectric Hidden Power Ice vs. 248 HP / 8 SpD Giratina-O: 136-160 (27 - 31.8%) -- guaranteed 4HKO
252+ Atk Choice Band Talonflame Brave Bird vs. 248 HP / 248+ Def Giratina-O: 169-201 (33.5 - 39.9%) -- guaranteed 3HKO
+6 252+ Atk Technician Mega Scizor Bullet Punch vs. 248 HP / 248+ Def Giratina-O: 346-408 (68.7 - 81.1%) -- guaranteed 2HKO
252 SpA Choice Specs Keldeo Icy Wind vs. 248 HP / 8 SpD Giratina-O: 178-210 (35.3 - 41.7%) -- guaranteed 3HKO
252+ SpA Mega Charizard Y Fire Blast vs. 248 HP / 8 SpD Giratina-O in Sun: 170-201 (33.7 - 39.9%) -- guaranteed 3HKO
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 248 HP / 248+ Def Giratina-O: 390-458 (77.5 - 91%) -- guaranteed 2HKO
252+ Atk Bisharp Sucker Punch vs. 248 HP / 248+ Def Giratina-O: 204-240 (40.5 - 47.7%) -- guaranteed 3HKO
252+ Atk Bisharp Knock Off vs. 248 HP / 248+ Def Giratina-O: 164-194 (32.6 - 38.5%) -- 97.9% chance to 3HKO
+2 252+ Atk Bisharp Knock Off vs. 248 HP / 248+ Def Giratina-O: 326-386 (64.8 - 76.7%) -- guaranteed 2HKO
252+ Atk Life Orb Excadrill Iron Head vs. 248 HP / 248+ Def Giratina-O: 140-165 (27.8 - 32.8%) -- guaranteed 4HKO
252+ SpA Choice Specs Adaptability Weedle Moonblast vs. 248 HP / 8 SpD Giratina-O: 3072-3616 (610.7 - 718.8%) -- guaranteed OHKO
Jokes for days.
252 Atk Landorus-T Stone Edge vs. 248 HP / 248+ Def Giratina-O: 85-101 (16.8 - 20%) -- possible 5HKO
I find Giratina-O in OU skeptical, but we'll see how this goes. I am leaning on pro-OU in the meantime since it may bring some good changes to the Meta.
inb4 Sableye-M is replaced by Giratina-O in Stall
252 Atk Tough Claws Mega Metagross Ice Punch vs. 248 HP / 248+ Def Giratina-O: 170-202 (33.7 - 40.1%) -- guaranteed 3HKO
252 Atk Mega Lopunny High Jump Kick vs. 248 HP / 248+ Def Giratina-O: 159-187 (31.6 - 37.1%) -- 83.8% chance to 3HKO
252 SpA Life Orb Landorus Hidden Power Ice vs. 248 HP / 8 SpD Giratina-O: 156-185 (31 - 36.7%) -- 65.2% chance to 3HKO
252 SpA Mega Manectric Hidden Power Ice vs. 248 HP / 8 SpD Giratina-O: 136-160 (27 - 31.8%) -- guaranteed 4HKO
252+ Atk Choice Band Talonflame Brave Bird vs. 248 HP / 248+ Def Giratina-O: 169-201 (33.5 - 39.9%) -- guaranteed 3HKO
+6 252+ Atk Technician Mega Scizor Bullet Punch vs. 248 HP / 248+ Def Giratina-O: 346-408 (68.7 - 81.1%) -- guaranteed 2HKO
252 SpA Choice Specs Keldeo Icy Wind vs. 248 HP / 8 SpD Giratina-O: 178-210 (35.3 - 41.7%) -- guaranteed 3HKO
252+ SpA Mega Charizard Y Fire Blast vs. 248 HP / 8 SpD Giratina-O in Sun: 170-201 (33.7 - 39.9%) -- guaranteed 3HKO
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 248 HP / 248+ Def Giratina-O: 390-458 (77.5 - 91%) -- guaranteed 2HKO
252+ Atk Bisharp Sucker Punch vs. 248 HP / 248+ Def Giratina-O: 204-240 (40.5 - 47.7%) -- guaranteed 3HKO
252+ Atk Bisharp Knock Off vs. 248 HP / 248+ Def Giratina-O: 164-194 (32.6 - 38.5%) -- 97.9% chance to 3HKO
+2 252+ Atk Bisharp Knock Off vs. 248 HP / 248+ Def Giratina-O: 326-386 (64.8 - 76.7%) -- guaranteed 2HKO
252+ Atk Life Orb Excadrill Iron Head vs. 248 HP / 248+ Def Giratina-O: 140-165 (27.8 - 32.8%) -- guaranteed 4HKO
252+ SpA Choice Specs Adaptability Weedle Moonblast vs. 248 HP / 8 SpD Giratina-O: 3072-3616 (610.7 - 718.8%) -- guaranteed OHKO
Jokes for days.
252 Atk Landorus-T Stone Edge vs. 248 HP / 248+ Def Giratina-O: 85-101 (16.8 - 20%) -- possible 5HKO
I find Giratina-O in OU skeptical, but we'll see how this goes. I am leaning on pro-OU in the meantime since it may bring some good changes to the Meta.
inb4 Sableye-M is replaced by Giratina-O in Stall