From the Final Fantasy Wikia found here:
Ozma’s exact nature is unknown, but because it is found within an “eidolon cave”, it can be speculated to be an ancient eidolon whose power the summoners couldn’t harness, and it was thus locked away.
Because Ozma is found within an “eidolon cave”, it can be speculated that Ozma, much like Leviathan, was an eidolon whose power the summoners couldn’t harness, and it was thus locked away.
Over the many years since, the legends that gave birth to Ozma would have vanished, and the people who used to tell them would be extinct, with the result that Ozma loses whatever form it may once have had and only appears to the party as a swirling mass of energy
As stated above, we find Ozma in an “eidolon cave”. This fact gives many fans reason, along with its immense power, to suggest it might be a lost summon.
Nothing in Final Fantasy IX’s lore that suggest eidolons can lose their form. However, even though Eidolons are magical entities, they don’t cast magic. They have magical attacks, but don’t necessary cast spells.
- Lv5 Death: Inflicts Instant Death on all party members whose levels are a multiple of 5. Ozma will only use Lv5 Death if the party has a member susceptible to it; if no party member is of a suitable level, Ozma will not waste its turn.
- Death: Inflicts instant death on one party member if it hits.
- Lv4 Holy: Deals Holy damage to all party members whose levels are a multiple of 4. Ozma will only use Lv4 Holy if the party has a member susceptible to it.
- Holy: Deals Holy damage to one target. Ozma will not use the ability if the target absorbs it.
- Flare: Deals non-elemental damage to one target.
- Flare Star: Deals damage based on the characters’ level. Can miss.
- Doomsday: Deals Shadow damage to all targets, including Ozma.
- Meteor: Deals random damage to the entire party, with the possibility of dealing 9999 to all of them.
- Curse: Deals non-elemental damage to all party members and inflicts Confuse, Poison, Slow, Mini, and Darkness.
- Mini: Inflicts Mini status on all targets.
- Esuna: Used if Ozma is afflicted with a status ailment.
- MP Absorb: Used if Ozma is low on MP. Drains MP from the party.
- Berserk: Used to counter attacks.
- Curaga: Used as a counterattack; there is a higher chance of countering with Curaga when Ozma’s health is low.
Most summons throughout the series do not cast magic on themselves or their opponents. These special attacks are magical in nature, however. Other superbosses throughout the series are also able to follow a pattern of attacks.
Whatever Omega Weapon actually is, it might not be an Eidolon, but rather just another (albeit difficult) super boss.