Its access to Spikes is great, and it has solid Special Attack and Defense stats as well. However, thanks to its only average offenses and plethora of weaknesses, it is hard-pressed to switch in safely. Its main switch-in opportunities will come against Choice Band users, but the likes of Skarmory, Regirock, and even Rhydon make for better counters to Choice Band Pokemon.
This sweeper set is Omastar's standard. Much like Kingdra, Omastar is a mediocre Pokemon that is reliant on the weather. Set up with Rain Dance, then fire off attacks: Ice Beam hits Dragon- and Grass-types while Hidden Power Electric takes down Gyarados; Swampert will fall in two or three hits anyway.
Lum Berry cures status, mainly paralysis, which can ruin a sweep. With a Speed stat of 198 and a rain boost, Omastar outruns all standard Pokemon, including Jolteon and Aerodactyl.
Omastar can take a defensive route too, but is hampered by its terrible typing. As with Cloyster: it can badly damages other users of Rapid Spin and Spikes, but isn't any good as a physical wall—if anything, Omastar is even worse than Cloyster in this role.
Thief is a cool move to steal Leftovers from Blissey, Suicune, Snorlax, or Milotic; this Omastar can pair with Tyranitar for more sandstorm damage. The other two moves should be used along with Leftovers. Protect can stall for additional Leftovers recovery and scout Choice Band users. This can significantly reduce prediction as, for the most part, they only have Earthquake and STAB Fighting moves with which to threaten Omastar.
As Omastar will not be tanking any special attacks regardless of investment, it should focus on physical bulk to sponge attacks from the likes of Salamence. For example, it can survive +1 Salamence's Earthquake, then KO it with Ice Beam.
Omastar's poor Attack stat means it cannot make use of the physical moves it gets. Toxic and Tickle can work decently alongside Spikes; however, Haze cannot be used with Spikes, which is a letdown.
Checks and Counters
Paralysis cripples the Rain Dance sweeper. Earthquake Snorlax and Blissey, as well as bulky Water-types such as Starmie, Suicune, and Milotic, can all take its hits easily and eventually beat it. Trace Porygon2 and Gardevoir counter the Swift Swim variant if they have Thunderbolt. Dugtrio can finish non-Swift Swim-boosted Omastar at about 60% health or lower. Just about any neutral special attack will OHKO or 2HKO Omastar, while any Grass-type move will OHKO it. However, note that not many Grass-types appreciate Ice Beam or a boosted Water-type attack, which is a plus for Omastar.