Ok, he's not really that.

If you want a detailed explanation on what Ozma is, go here:

http://www.gamefaqs.com/console/psx/file/197338/36121

If you are too lazy to read it, basically, it says that Ozma is a long dead, or forgotten Eidolon. Since it said in the game the Eidolons were born from and took the form of the way people believed from legends, it is assumed that Ozma lost his original from, (or rather, took the from of a swirling ball of death) because the memories of people who still remembered his form were long gone and those people were probably long gone too.

Also, it is assumed that Ozma was dead too, since the Bradygames FFIX Strategy Guide says that the thing that you touch to fight Ozma was an "Eidolon Grave." (Then again, the guide wasn't too good in my opinion; after all, that's where the whole "Ragtime Mouse" thing came which people said instead of the game's name, "Ragtimer")

In short, he was the most powerful Eidolon (though arguably Alexander might be stronger) that was long forgotten/dead.

He is also the toughest boss in FFIX and in my opinion, the toughest of the whole FF series.