This bastard is the hardest boss in the game, and is of course optional. In fact, it's quite time consuming to even be able to get to him. You have to do a majority of the Chocobo sidequest. I debated uploading this after the final boss, but I figured I'll do this now and finish with the final boss.
Note: I'm a higher level in this than I will be in the final boss fight, since I recorded this post-game. See stats at the end.
What makes him hard is how he chains attacks together. He'll spam Curse on you like there's no tomorrow and then randomly hit you with a Meteor after which may or may not OHKO you. His arsenal is just devastating. I do not recommend taking on Ozma if anyone is under LV50, even under LV60 is tough. Hell, walk into this at LV99 and you can still Game Over rather easily.
Preparing for Ozma
Preparing for this is immensely important. The absolute first thing you'll need to do is the Friendly Monster sidequest. It's a sidequest where you find about 13-14 friendly monsters randomly around the world in certain areas and when you encounter one. You'll want to FAQ it, although you may have already ran into around half of them through regular gameplay.
When you do the last one, he'll say you can "reach the round guy now" or something. It means you'll be able to physically attack Ozma with regular attacks. However, that's not the true benefit you get from this. What they don't tell you is that now Ozma's Doosmsday attack will now hit him for 9999 damage instead of heal him for 9999 while massacring your party only. This is greatly helpful and I can't imagine going through the pain of doing this without it.
Next, you'll want to level up to at least LV60 IMO. Even LV70 would be a good idea. Grand Dragons & Yan's are a great way to level up at the end of the game.
For equipment, you're going to definitely want Dark resisting armor. You can buy some good end game equipment that does it in various places, most likely Daguarro or whatever. Doomsdays is a dark attack and will do tons of damage if you just let it hit you fully. That's about it for important equipment, though.
For abilities, you're going to want the majorly bad status ailments guarded, since Ozma spams Curse. You don't want anyone being hit with anything that stops them from being able to use a move, or else you'll probably game over.
Last, but not least, let me give you a huge tip. A borderline broken combination of two abilities in this game is Auto-Haste and Auto-Regen. Auto-Haste by itself is godly. Auto-Regen alone isn't too helpful, but if you have Auto-Haste on as well, the Regen rate will be faster and at high levels you can recover 200-500 HP every couple of seconds! That's insane when you consider some of the long ass animations in the game, you could be fully healed from 1 HP from some. So if you have enough AP, definitely keep those two. Return Magic is also another good one if you did the Friendly Monster sidequest because of Doomsday if you can guard against it.
Strategy
Don't expect to kill Ozma traditionally. He counters most hits you do to him, a lot of the times it'll be Curaga to heal him for 9999. So hitting him with a 4000 hit physical is going to get you nowhere. Instead, abuse the fixed 9999 moves. Stuff like Freya's Dragon's Crest and Zidane's Thievery are great ideas. Stiener's Shock is great as well...except when you get hit with Curse (which inflicts Tiny), then it does garbage damage instead of 9999.
Ozma's scariest combo is Curse + Meteor. Curse won't kill you, but you'll be hurting after it and Meteor can OHKO EVERYONE randomly. I'm not sure why the damage is so random, but more times than not you'll die from it. So you'll have to hope it misses at least one person. But he's known to use it after Curse sometimes, which can be an auto game over for many. (Think: Mysterious Figure with double Collision Magnet move).
Other than that, the rest of his attacks aren't too bad alone. He has LV5 Death, LV4 Holy, Absorb MP, Flare, Holy, Esuna, Mini, and Death for other regular moves.
His counter attacks are either going to likely be Curaga, or Berserk on the person who attacked him. Berserk is most annoying, if you let the Berserk person keep hitting Ozma he'll just heal for near 9999 practically every time.
Overall, this is an extremely difficult fight. YOu need pretty specific strategies to take him out. There are a few broken strats out there to make this trivial, but that's about it. Out of FFI-FFX (Minus FFV Omega), I consider Ozma the hardest optional boss due to how random he is and how hard he can hit you with his chained attacks.
That's about all I can mention in a YT description. Read the Ozma FAQ on GameFAQs for more in depth strats if you're having trouble. With a few very specific setups, he can be beaten at LV1 and even soloed with a couple chars.
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