The Omega Weapon is a Keyblade that can be wielded by Roxas and Xion in Kingdom Hearts 358/2 Days. It can be upgraded to the Omega Weapon+. With the absence of Ultima Weapon, this Keyblade fills the role as one of the strongest weapons in the game.
The Omega Weapon's handle is white and its circular guard has gold outer edges and black inner ones. The guard is not a complete circle, as there is a thin gap on either side of it, splitting the guard into an upper half and a lower one. The top of the guard is decorated by three red diamonds, and there is a heart-shaped gap in it just above the handle. The shaft is black, has gold, concave edges, and lined by small, diamond-shaped gaps.
The head of the blade is a thin, incomplete, gold ring with a spike on its tip. The teeth are comprised by a gold arc with three spikes of the same color lining its outer edge. Though the teeth are connected to the top of the shaft, they are also held onto the head of the blade by three large, black spikes that point inward. The Keychain token is a vaguely cross-shaped symbol with a heart-shaped gap in its center. The top half is gold and the lower half is grey. The symbols that appear when hitting an enemy is similar to that of Terra, Aqua, and Ventus's Wayfinders; however it is lacking the heart in the middle, and is colored gold and black.
All of Omega Weapon's passive abilities are connected to the player's HP. Having a full HP Bar will cause the Omega Weapon to deal more damage to enemies and make them stagger with each hit. A full HP Bar also reduces the damage taken by the enemy down to ⅓ of the usual. The ground combo starts by jumping back, then rushing forward while performing a diagonal spinning slash with the Keyblade, followed by an upward vertical slash from the left leg, and finishes by jumping into air, leaning into a slight angle, and doing a spinning slash. Pressing will cause a downward thrust to the ground.
For the aerial combo, it causes a vertical downward slash, then rushes forward while leaning on an angle, and performing an upward spinning slash on the right side, and it ends with a downward spinning slash on the left side.
- The Omega Weapon is a recurring superboss in the Final Fantasy series that is usually connected to the Ultima Weapon, providing an analogy to Sora's Ultima Weapon.
- The abilities granted by this weapon are also a reference to the Ultima Weapon in the Final Fantasy universe, where it gains more strength as the user gains more HP.
- Although released later, Final Fantasy XIII features Omega Weapon as the ultimate form of all Lightning's weapon models.