• R/B
• G/S
• R/S
• D/P
• Strategy
• Moves

Omastar

Mimic is for switch in Starmies, as Mimicking Thunderbolt is awesome. You ought to use Rest and Seismic Toss in UU. Seismic Toss is good for damaging Water-types, but don't try and take most of them out 1 on 1, because Omastar doesn't resist Water attacks. Whether to use Surf or Hydro Pump is a question between accuracy and power; additionally, Surf boasts three times as much PP as Hydro Pump. Omastar absolutely owns all those silly Pokémon that can only use Normal or Fire moves, and is a great addition to all UU teams.

Other Options

Body Slam is useful for the chance of paralysis, but the non-STAB damage will be minimal. Leer in conjunction with Body Slam could also be useful, but Omastar is basically a one-trick pony. Reflect is a possibility to aid in tanking. Horn Drill can take out Chansey, but only use this if OHKOs are allowed.

Opinion

Basically, there is no substantial reason to use Omastar in standard, because it has common weaknesses and is rather outclassed. In UU, however, it's a great Pokémon. It has fair HP, great Defense, and amazing Special.

Counters

Poliwrath will switch in and use Amnesia to dominate Omastar, as well as possibly hitting it with STAB powered Submission. Golduck can switch in and use Amnesia as well. Tangela, although it fears Omastar's Blizzard, has huge Special and can Mega Drain it for huge damage. Electabuzz can also significantly damage Omastar. Kingler does not get damaged much by any of Omastar's moves, and while it uses Normal attacks and does not have a giant Special, letting it get multiple Swords Dance boosts is not a good idea. Machamp can hurt Omastar with Earthquake and Submission and the two Pokémon have equal Speed, so while Omastar has good Defense, it is fairly risky to stay in.