Nooo, chillen, plz, calm j00rselfs doun. I was wondering what he meant by Omega Weapon too, figured it was Ultima, but then he said Earth Harp and I was like WTF? That's Emerald! And so it is, as seen by the underwater business.

There are five WEAPONS in FFVII. Some spoilers may ensue, depending on your location in the game and disposition on things in general (Such as what IS a spoiler.) - you have been warned.

Diamond WEAPON - The big white dude who stomps towards Midgar. Big sod kills one of my fave characters ;__;
Sapphire WEAPON - This thing's name isn't confirmed, as you never fight it, and it is never referred to. However, general consensus seems to be the things name would best be Sapphire. He's the sea-monster one who attacks Junon.
Ultima WEAPON - The big dragon-ish guy who flies around the place, and attacks Mideel. Surprisingly weak. You get Cloud's ultimate weapon, the Ultima Weapon, for beating it enough times (Ram it with the Highwind until it stops moving, then ram it again for a fight. Once if hovers over the jelly-like forest near Cosmo Canyon, it's the last fight. Killing him also opens a gold-chocobo free route to said forest.).
Emerald WEAPON - Big green tough SoB who swims around underwater. You need the underwater materia to gets him, and you also need to know what you're doing. I do, but I wouldn't want to spoil the trick for others who mightn't yet know. You get an Earth Harp, which taken to Kalm will yeild those lovely mastered materia.
Ruby WEAPON - One word: Scum. Well, worse ones spring to mind, but the lady wants me to watch the swears, so scum it is. I find him to be by far the hardest, I've only beaten him once, and that was through sheer luck. You get the Desert Rose, which'll get you a Gold Chocobo. Anyone who doesn't already have one is a God of FFVII, 'cause I have no idea how to beat Ruby without KotR. And what're you meant to do with a second one, cook it?

And that concludes my short discourse on the WEAPONS of FFVII. No, there is no Omega WEAPON.