Ou basically does all of the boss-specific percentage based mechanics from the base 5 Omen bosses, 1 time each, specific moves used at set percentages of Ou's HP between 95% and 30%.
See list on the BG page for Ou.
Ou gets unique moves at 15% and 10%. 15% is Gardez, an AoE terror move. You want to trigger that, make sure everyone gets un-terrored and ready for a last zerg, then CAREFULLY bring HP down and prepare for a mad rush to take Ou from 10% to Ou death. At 10%, it will use Prophylaxis - this recovers 10% of Ou's HP (so, up to ~20% total) and sets a 30 second timer in which you must kill Ou... or else it recovers to 100% and will reuse all %age based abilities again.
Keys are:
- Ou takes extra SC damage when Prophylaxis is active, so using a strong multi-step SC is very effective.
- BEFORE you get to 10%/Prophylaxis, have 3000tp on any DDs and be sure they pop any JAs before 10% so you don't lose any time with JA delay during the crucial 30 second rush.
As for setup, it's pretty similar to the other Omen caturae. If you can do all of them, you can do Ou. There's also no need to deal with some of the more limiting caturae gimmicks like Kei's magic stuff or Kin's recovery if hit during JA though. For the most safety on Ou, bringing two tanks can be helpful for hate reset moves and dealing with possible death from Dancing Fullers/Unfaltering Bravado (we tend to bring a PLD and a PUP, and just let automaton eat the nasty one-shot kill moves). My group that typically does Omen with 6-12 players generally does Ou with a pretty standard ranged setup: tank(s)+WHM, then something like RNG RNG COR GEO and add additional ranged or support jobs as available (additional GEO/BRD/COR/RDM/etc.). No reason you can't do it with SMNs or melee, and while melee style is not impossible there's more margin for error than ranged due to higher DD gear requirements and more need for healing.