1. Rain Dance
2. Surf/Hydro Pump
3. Ice Beam
4. Hidden Power Grass/Electric

Tier:
UU
Item:
Life Orb
Ability:
Swift Swim: This Pokemon's Speed doubles during Rain Dance.
Nature:
Modest
EVs:
4 HP - 252 Sp.Atk - 252 Spd

Omastar can make a great sweeper in the lower tiers. It has high Special Attack and his speed can become very fast with Rain Dance. He will need Swift Swim and Rain Dance to become a threat though.

First off you will use Rain Dance to be able to outrun most Pokemon you will face. You will then start attacking with your water move and Ice Beam. Hidden Power is a filler to use against a couple Pokemon depending on what you need. If you don't want to use Hidden Power you can use Ancientpower instead.

Omastar wants to deal as much damage without taking a lot of damage, but Omastar can still take a hit with its great Defense.

Omastar Pokedex Page