Omega (XS1)

 is Disc 2, Track 15 on the Xenosaga Original Soundtrack and Disc 1, Track 15 on the album. ('Omega' is an unofficial translation, as seen in the VGMdb listing of the album.)

It plays in the cutscene where Wilhelm's fleet joins the fight against the Gnosis around the Kukai Foundation. It also plays very briefly immediately before the Gargoyle and the Tiamat battles.