To reach Ozma, you have to complete most of the chocobo side quest that begins in the Chocobo forest near the beginning of the game --- to speed you along, see this list of Chocographs which will help you find the treasures and evolve your chocobo to a Gold Chocobo. Once you have a Gold Chocobo and can reach the Chocobo Air Garden, you are able to fight Ozma. Defeating Ozma isn't easy, however.
Ozma is the hardest battle in the game. If your characters are less than level 60, you will probably not be able to defeat Ozma. If your characters are above level 60, you're still going to have a pretty rough time of it. But reaching Ozma is easy, at least. Once you're in the Chocobo Air Garden, simple go to the rock on the leftmost ``island'' and search around until a ``?'' appears. Press X here. Mene will warn you twice that he senses something other-worldly and that he doesn't think you should continue. You will be given a chance to select your party, re-equip, and even back out of the fight; however, the menu where you can reselect your party is your last chance to cancel.
Once you proceed from there, you will begin the battle with Ozma. After that, how hard can it be? Ozma only has 50,000HP which is small compared to the Weapons in FF7 and FF8 which had 500,000-1,000,000HP. How bad can it really be? I'll let you find that out for yourself :)
You should seriously consider meeting these prerequisites before attempting Ozma.
Ozma's major characteristics are listed below:
Note that Ozma has two sides, a light side and a dark side. The side that is on top determines which magics Ozma is vulnerable and which ones Ozma will absorb. When the dark side is on top, Ozma will absorb Holy elemental attacks, but is weak against Shadow elemental attacks. When the light side is on top, Ozma will absorb Shadow elemental attacks, but is weak against Holy. Ozma rotates periodically, so it is hard to plan attacks to take advantage of the vulnerabilities; and with Ozma, you usually only get one chance.
This is particularly noticable when Ozma casts Doomsday. Note that Doomsday hits everyone currently on the screen (it may exempt Freija if she has already jumped), therefore if the right side is up Ozma will take 9999HP damage just like every party member that isn't guarded against Shadow.
Listed below are Ozma's attacks which can be particularly devastating.
NOTE: None of Ozma's magic is reflectable. Also, Ozma is immune to reflect.
On 2001.03.26, I finally defeated Ozma as part of my completionist game game for FFIX. That was one fun battle too, given the (lack of) leveling up I did -- after about 40 minutes, I lucked out when Zidane entered Trance, after that it only took 1 Doomsday and 3 Grand Lethals -- too bad the Doomsday knocked out the rest of my party... Oh well, can't have everything when you don't level up...
If you defeat Ozma before finding the Master Synthesist, Hades, Hades will begin the battle with one of the more amusing battle dialogues I've seen: ``Wh-What!? You've defeated Ozma!? I've come too far to retreat. Onward!''
On 2002.04.12, I defeated Ozma a second time during a replay. Again, I did not level up my characters more than necessary to learn particular skills. Notable in this game: I did not find all of the land spirits in this game. As a result, I could not reach Ozma; this makes the battle far more interesting. Zidane was susceptible to Lv5 Death, which added a bit of challenge. This attempt did not require much time; I mainly needed to figure out how to use each character, and then wait for a round where Ozma didn't begin with Meteor.
For this battle, I used the following characters and equipment:
Characters were equipped with auto-haste, auto-potion, auto-regen, antibody, and clear-headed whenever possible. All other abilities were icing on the cake.
In this battle, Ozma began with Doomsday which was an all-party heal. Zidane merrily pilfered items from Ozma until he entered Trance; then I started using Grand Lethal (9999 HP damage). Steiner used Shock (9999 HP damage when he wasn't mini), and Freija used Dragon's Crest (~5000 HP damage). Freija's attack was equally effective when she was mini, which helped. In the end, a meteor and death knocked out all but Zidane and Steiner, but... Grand Lethal was enough to defeat Ozma.
If you defeat Ozma before finding Yan (the friendly land spirit, not the less-friendly deadly enemy), the conversation goes something like this:
Give me a Diamond.
Give Yan a diamond
Thanks a lot!
Reach the round guy...
You already beat him?