Ozma has around 60,000 HP. You can attack him without doing the sidequest, but only with magic attacks. physical attacks require the sidequest.
TAKEN FROM OZMA FAQs ON GAMEFAQS:
Ozma has several spells that can wipe you out. He has some other annoying
spells that can also pretty much wipe you out. Then, to spite you, he'll
cure himself. Oh yeah, can't forget his tendency to take two or three
turns at once, and his speed is high enough that he can go pretty much after
every single one of your party's moves. Here's a list of what he's got:
Death - Standard Instant Death spell, works most of the time
L5 Death - same as Quina's, if you're level is a multiple of 5, you're dead.
MP Absorb - if Ozma somehow gets low on MP, he'll use this on someone and
replenish his supply.
Star Flare - Old school (FF6, if I remember correctly) attack-all spell,
does roughly 2500 damage to all. Tends to miss people
sometimes, though.
Curaga - Oh, he'll cast this on himself plenty of times. He heals for
about 7000 a shot.
Mini - This one is just plain annoying. All your characters are, most of
the time, afflicted with the Mini status. Sometimes it'll miss one
person.
Doomsday - Remember Merton from FF6? Well, this is similar, except sometimes
it heals Ozma, sometimes it hurts it. With your party's equipment
setup, it always heals you. Pray for this spell a lot. (WITHOUT SETUP WILL KILL YOU)
Curse - Pray that you never see this spell. It causes every status ailment
in the book unless you've guarded against it, and that includes
Blind, Mini, Confuse, Poison, all your favorites. On top of that,
it can do enough damage to wipe your party out in one shot. The
damage seems to be somewhat random, though.
Flare - Just like Vivi's although it hurts more. Much more.
Holy - Just like Eiko's. Doesn't hurt much more. I considered it a breather
when he cast it on me, cause he has so many better spells.
Meteor - If he casts this, you're toast. Well, mostly. I can't figure out
why it'll do 9999 to a character one time, then 800 the next, and
then miss the next time.
Ozma's Stats:
HP: 65535
MP: 9999
so yeah......