This thing is a LOT harder than any FF boss I've fought, including the weapons and Sephiroth,they've nothing on this thing. You really don't have to unlock it, in the Air Garden, go to the Second Island, up to the hole, and press X. The moogle will fly over and warn you and you'll have a chance to prepare before you fight it. Anyway, it took me MANY tries to beat this thing, but here's the strategy I used:
Using Freya, let her kill a lot of dragons, make sure she has the last blow, after she learns the ability Dragon Crest. Keeping killing dragons with her till the ability does at least 7,000 damage. Then, fight and fight with Zidance until his trance meter is so high that in one hit he'd go into trance and then make sure he's not hit again, you'll need the trance to fight Ozma. I don't remember if summons do a lot of damage to Ozma, but take Garnet anyway when you go to fight it, it's a good idea to take both Eiko and Garnet to heal or you could just take Vivi, he has an attack called Doomsday that will either heal or damage Ozma, but if you take Vivi, it's always a gamble each time you fight it. Doomsday might heal it and you die, but then the next time it might damage when you fight Ozma again. It's also not a bad idea to take someone with high physical strenght with you, but either way you will need at least one healer, like Garnet and Eiko. Also, Quina can learn this technique called Mp Hammer I think that's what it's called and if you use this on Ozma, it will make the battle so much easier because the technique destroys all the opponent's MP so then Ozma won't be able to cast any spells. Anyway, it's also not a bad idea to make everyone immune to all status problems or have auto regen ability put into effect and have your beat armor and weapons on the party you use to battle Ozma. When you enter the battle, Ozma will attack of course, sometimes it'll use doomsday the first time and doomsday does a lot of damage, sometimes when it uses doomsday it does 9999 to itself, but most of the time my party died the minute they were hit by it. Anyway, if it hits Zidane and his trance meter is high like I told you to put, then he'll go into trance. Use Garnet and Eiko to heal and with Zidance use his trance attacks. If you have Quina and it knows that Mp hammer technique, then use it but if you have Vivi, the best thing you can do is use Doomsday because all the other spells don't do much damage at all to him. Using Freya, use Dragon Crest. When I finally beat Ozma, Doomsday was doing damage to it and Ozma used it a lot, so with Zidane doing 9,999 and Freya doing 6000 to 7000 and Ozma doing 9999 to itself, I finally beat it and got the Ozma card. Good Luck.