Of all the super bosses in FFs from FFV to FFX, Ozma and Emerald Weapon are the ones that I can't beat 100%, meaning that even the best strategies I use for them don't guarantee victory.
- My best bet for Ozma is Quina, with her super useful 'Angel's Snack' blue magic, curing all you status ailments to counter against Ozma's Curse.
- Then Eiko, because of her magic status and Curaga (and that chance that Pheonix will revive your party if you're screwed).
- And then Steiner for Shock (guaranteed 9999 damage if your level is high enough). If Steiner (or Zidane) gets berserked, don't panic. They'll still do damage, and Quina's Angel's Snack will take care of the rest.
- Then Zidane, because you can't remove him :\
* Every single turn for Eiko, always use Curaga, no matter what, but do time yourself - cure the party AFTER Ozma unleashed any of his attacks, no point curing them when they're in full health.
Quina: either angel's snack or magic hammer, depleting heaps of MP from Ozma. Use Steiner's shock all the time. To better you chances, have Eiko auto-hasted somehow (I forgot what to use, but it's there if you synthesize stuff I think) so that you can cure more often. Zidane just attacks normally (remember to finish the friendly monsters quest). If Steiner (or Zidane) gets berserked, don't panic. They'll still do damage, and Quina's Angel's Snack will take care of the rest.
Hopefully, Ozma's Meteor isn't devastating throughout the battle... If things get really really bad, use elixirs wisely - only to those who may die after Ozma's next attack. Towards the end, he'll use cure big time everytime you hit it. If Zidane can't deal 9999 damage, then just leave the damaging to Steiner - it won't be long before Ozma falls.
Oh yeah, try to equip most of your characters with something that absorbs darkness (Pumice Piece, I think?) to benefit from his Doomsday attack.