Level 100 Statistics (see level 5, 50, 100)
Spikes are great, Special Attack and Defense are great. I suppose it is arguably tied with Gorebyss as the best Rain Dancer, and it might get some Spikes in. The typing is what it hurts the most.
The "sweeping," standard Omastar. Can demolish UU teams. Get a Rain Dance in, fire away from your blasting high Special Attack, Ice Beaming Dragons and Grasses. Quagsire is more of a threat than Mantine is (it is used much more there), so use Hidden Power Grass for that. Lum Berry is to cure status, mainly paralysis, which can ruin a sweep.
Defensive Omastar. Too bad its typing sucks so much. In the same kind of Spiker category as Cloyster: it damages other Spikers and Spinners badly, but isn't any good as a physical wall, though Omastar is probably even worse. Thief is a cool suicide move, stealing Leftovers can work in your advantage. Toxic and Protect work with Leftovers. Protect can stall time for you to recover with Leftovers, and it can scout Choice Banders, since for the most part, only Earthquake is going to hurt Omastar from Choice Banders (and STAB Fighting moves), so this really eases up prediction.
Not much to see here, Omastar doesn't get Rapid Spin, Explosion, or a good Attack stat to use the physical moves it gets, and still it doesn't get much of that. I suppose Toxic and Tickle can work for Spikes abuse, a very little bit. Haze cannot be used with Spikes, and that's a letdown.
For the Rain Dancer, you need about 181 Speed (140 EVs), as you are faster than all but Electrode after a Rain Dance in UU. Then max out Special Attack, and put your leftover EVs in HP.
The other set needs maximum HP, a bit of Special Attack, and the rest in Defense. You're not taking any Special hits anyway, or if you are, you are dead.
Checks and Counters
Quagsire is actually healed by your Surfs, so if Omastar doesn't have Hidden Power Grass, it can have some difficulty. Mantine stops it if it doesn't have Hidden Power Electric. Other Waters can usually survive it and Surf back for the KO, like Golduck. For the rest, most UU Pokémon will take a big beating from this.