Final Fantasy V has its own mythology of Monster vs Mech. It’s unclear who built Omega V. No one knows why he’s in the Interdimensional Rift. But Omega Weapon’s so powerful, the Sealed Weapons can destroy it. Also in the Inter-dimensional Rift, trapped in a box, is Shinryu; the Sealed Weapons could not defeat the dragon either. According to the Thousand-year-old book in the Rift’s library:
“The Roiling skies let loose a vile beast without a soul; Omega was its name. The dragon Shinryu came upon its heels. Not e’en the legendary weapons twelve were able to subdue these evil beasts. Inside the Rift the demons were interred; so should they stay until forever’s end, their names to stay unspoken evermore.”
None of the Sealed Weapons could defeat either Omega or Shinryu, because they do elemental damage. Elemental weapons cannot harm either entity. For Omega, this might be by design. I think Omega’s sole purpose was to defeat Shinryu. Since Shinryu does primarily water damage, it would make sense for Omega to be water resistant.
Omega’s signature move, Wave Cannon, does heavy non-elemental damage. It would have been more than enough to defeat Shinryu after a couple of rounds. And consider its construction (round body, four spider-like legs) could have had enough mobility to outmaneuver Shinryu.
So then why doesn’t Omega defeat Shinryu? It may have the same malfunction that WarMech may have had. We Bartz and the others encounter it, it does nothing but aimlessly wander around. Its detection is surprisingly low, to where the party can simply walk past it.
Seeing that both entities reside in the Void, they find themselves in other Final Fantasy worlds, much like Gilgamesh. In Final Fantasy XII, Omega Mark XII is a war-machine; the party can only battle him when they take the Yiazmat Hunt. It’s possible that both Omega Mark XII and Yiazmat are Omega and Shinryu from the other dimension.