Get equipment that absorbs Shadow.
I used Zidane, Amarant, Eiko and Freya.
Zidane - Thievery: 9999 damage.
Eiko - Healer. Her Phoenix summon can be very useful. Holy can also do some nice damage if she gets to use it. Full-Life is also very useful.
Amarant - He can throw stuff at Ozma. Aura and Chakra can be fairly useful too. Steiner's a pretty good substitute as well; Shock works well.
Freya - Reis's Wind and Dragon Crest. Reis's Wind to regenerate HP and Dragon's Crest to do 9999 damage.
I did the side-quest that enables you to hit him with close-range attacks. I was level 70+ when I beat him.
Ozma is all about luck, as was stated in the 2nd post above mine.
[ April 10, 2004, 12:51 PM: Message edited by: StarryPeach ]