Omastar can give some hurting. Thanks to the suckification of Tentacruel (lowering its Special Attack 80 points x.x), Omastar has the highest Special Attack of all Water types, not to mention 348 Defense. However, its 238 Special Defense hurts, especially if you're fighting a Grass. On the bright side, it has more HP than Starmie ^_^
Its 208 Speed lets it get the jump on most popular Ground types (Rhydon/Marowak). And I'm sure they'll love getting blasted in the face by a 328 SA STAB 95/120 base move, depending on whether you like to take risks.
I suggest the following set:
Omastar can't do much offensively (its Ancientpower is weak considering its 218 Attack). Rain Dance pumps up Hydro Pump/Surf's power to the point where a single Hydro Pump can OHKO Tyranitar. That's saying a lot. Unfortunately, Tyranitar is faster, and can blast you with 2 Quakes before you know what hit you. Ice Beam is if you get extremely lucky and face a paralyzed Jumpluff or something. Horn Drill is nothing more than a last resort, especially against Curselax.
I probably made a total fool of myself for making the stupidest Omastar moveset ever, but I am known to be an idiot sometimes.
"Aww, does the poor widdle Death Stawmie need mommie to kiss away its boo-boo?" - White Cat
"Ugh, that's impossible!" - R-2 Rival, after Belly Drum Lax OHKO'd Mewtwo with Double-Edge