Omastar @ Life Orb / Focus Sash
Role: Sp. Atk Sweeper
Ability: Weak Armor
EVs: 4 HP / 252 Sp. Atk / 252 Speed
Modest Nature (- Atk , + Sp. Atk )
- Shell Smash
- Surf / Hydro Pump
- Hidden Power [Grass]
- Ice Beam / Earth Power
=== EXPLANATION ===
Unlike most White Herb + Shell Smash combinations, Omastar doesn't really need White Herb because of its low defenses. It's most likely going to be one-shotted with a strong super-effective move, like Close Combat or Energy Ball (even if you have White Herb). Focus Sash is much more effective in this case. Life Orb is also a possibility for boosting its damage, but you may not always get to set-up Shell Smash.
Surf and Hydro Pump are both high-power alternatives, but Surf is generally preferred for its good damage and its 100% accuracy. You shouldn't be using Scald because it slightly lowers Omastar's power and a sweeper like Omastar doesn't need to have the burn chance.
Hidden Power [Grass] is always a necessity because it grants that bulky Water-types do not wall Omastar.
Ice Beam is generally preferred for dealing with annoying Grass-types. Earth Power does see some usage in dealing with Electric-types, but it has limited usage outside of that.