You could always get Steiner with Shock, power up Freya's Dragon Crest to the max, power up Zidane's Thievery to the max, and power up Quina's Frog Drop to the max. Four characters with instant 9,999 damage attacks. Shadow absorbing armor helps with keeping you alive, but with that much damage at your disposal, you should be able to pound Ozma into the ground easily.

I think I used Zidane, Steiner, Freya, and Amarant. I just kept spamming steal with Zidane to get the items off of Ozma while Steiner and Freya wailed away with Shock and Dragon Crest. I equipped Amarant with the healer ability and had him attack my own characters for 6,000+ HP restoration. If I needed him to attack, I could throw out a No Mercy for about that much damage. I didn't have any shadow absorbing equips on, so I think I had a considerable amount of luck when I beat Ozma. I haven't tried again since, and it was so long ago, so meh.