Omega Weapon is an optional boss in Final Fantasy X and is the fearsome fiend transformed from Omega through his anger. Its anger was so great, it spawned the lesser Ultima Weapon. The words "Omega Weapon" are inscribed in Omega Weapon's tail in Spiran script.
In the original North American and Japanese versions, Omega Weapon has mere 99,999 HP, not much of a greater challenge over Ultima Weapon. It was upgraded for the International, PAL, and HD Remaster versions, now boasting 999,999 HP; it could thus be considered a superboss.
Omega Weapon's normal physical attack will deal around 4,000 HP damage, and will also inflict a variety of status effects such as Petrify and Confuse. Core Energy is an upgraded version of Ultima Weapon's and will cause between 4,500 and 9,999 HP to one character. Shimmering Rain also returns and will hit the whole party for around 2,500 damage. Its ultimate attack is Nova, which can deal over 7000 damage to all.
Shell, Haste and Protect are important as Omega Weapon cannot be inflicted with debilitating effects, such as Delay, Slow or Break attacks. The special skill Quick Hit is useful to get more turns.
In the NTSC version, Omega Weapon will not put up much of a fight. The player's goal should be to deal as much damage as possible in a short time, which is made possible by the use of Celestial Weapons, Break Damage Limit, Overdrives, aeons, and Trio of 9999 (though none of these are required to defeat Omega Weapon). It's likely Omega Weapon will be defeated before it will use Nova.
In the International, PAL and HD Remaster versions, Omega Weapon's additional HP and elemental absorbency make it harder to defeat and Tidus's Brotherhood shouldn't be used because it is Water-elemental. Mighty Guard, either from Kimahri or Rikku, is a major help, as one only has so many items and so much MP, and they may not last long enough to deal with Omega Weapon's nearly one million HP. On the other hand, Osmose now works against Omega Weapon.