It's just nuts how words can have more than one meaning.
Anyway. Making a rare attempt to get a theory thread back on track without being 12 about it. It's late and I always try to do this shit when I'm way too tired, but yolo.
I think Omega is a mobile factory that early human colonists brought with them. Its main power is shooting dozens of shitty bombs that are obviously not military-grade, so it's not a weapon. Omega is only manufacturing the deadliest thing it can, which are probably not actually bombs. Some kind of fragile, exploding drone, perhaps.
Omega uses its mouth to dig out areas for construction, then nanobots inside Omega turn the earth it ate into useful materials. Machines like Omega, teamed up with construction workers, could turn solid rock into housing (like Grasstown) very quickly. Omega is autonomous enough to decide what to build if the need arises, as demonstrated by its spamming of round floaty bullshit when Quote fights it. Or maybe Omega was designed to build round floaty bullshit, who can tell with humans in the Cave Story universe.
Omega has legs because those are one of the best ways to traverse rocky ground, which cave floors usually consist of. An array of treads like Monster X has would be more effective, but slightly less cool, so legs it had to be. Omega might also have been designed to do heavy lifting, if it possesses enough power to jump ten feet in the air.
This doesn't explain why humans wanted their factories to make autonomous decisions, or why their factories would decide to attack innocent robots, or why blowing Omega up moves the sunstones, but it's a start.