Omastar is capable of being a strong threat in OU thanks its impressive base 115 Special Attack and access to Shell Smash, but there's quite a bit holding it back. Foremost, the prominence of powerful threats such as Keldeo, Thundurus-T, Kyurem-B, Landorus-T, and Garchomp makes even switching in a difficult task for Omastar. To make things worse, even at +2 Speed, Omastar risks being outsped and revenge killed by every relevant Choice Scarf user in the tier. All of this, in addition to a lacking movepool, keeps Omastar at the bottom of the barrel, facing stiff competition with even Gorebyss. Omastar's saving grace lies in the flexibility it has fitting onto teams; thanks to its Rock typing and Dream World ability, Weak Armor, Omastar can find a comfy spot on many different offensive teams. Though Omastar matches up poorly with many key threats in OU, it's rare to come across a check to rain teams that can keep up constant offensive pressure. Working past Omastar's weaknesses will definitely come with a huge payoff; persevere and it will not disappoint.
This set's aim is simple: use Shell Smash at the appropriate time and mow down any Pokemon unlucky enough to get in Omastar's way. Omastar's respectable defenses and threatening base 115 Special Attack help it achieve this, giving it numerous opportunities to set up. Ice Beam and Hidden Power Grass offer Omastar the best possible coverage, preventing Water resists from prematurely ending Omastar's sweep. The choice of Water-type STAB ultimately boils down to the plan for how Omastar is to be used; Surf is preferred for cleaning up and sweeping in general, while Hydro Pump gives Omastar greater wallbreaking potential. Likewise, Omastar's held item should reflect its intended purpose. White Herb preserves Omastar's defenses after a Shell Smash, making it much more reliable for sweeping. Life Orb, however, helps Omastar power through some major defensive threats, namely Ferrothorn and Chansey. With rain support, a boost from Shell Smash, entry hazards, and a Life Orb attached, Omastar has a chance to OHKO both with Hydro Pump. Regardless, Omastar will dish out massive damage after a boost, so make the choice that helps Omastar synergize best with its team.Team Options & Additional Comments >>>
Unlike most Pokemon introduced in RBY, Omastar's movepool is rather shallow, leaving it with very few viable options. Omastar can set Rain Dance up for itself and its team. This can be particularly useful when combating other weather starters; Ninetales, Tyranitar, and Hippowdon will fall to a rain-boosted Hydro Pump, though Abomasnow will require a bit of patience to take out. Like nearly every other Water-type, Omastar has access to Scald, but it is better off simply powering through the opposition with a noticeably stronger Surf or Hydro Pump. AncientPower is Omastar's only available Rock-type STAB aside from Hidden Power Rock, but its low PP and Base Power make it overall inferior to Ice Beam. Omastar can forgo Shell Smash for Rock Polish, as it is sometimes powerful enough to sweep without the Special Attack boost. The Special Attack boost from Shell Smash will be sorely missed, but Rock Polish comes with the benefit of retaining Omastar's good defenses without the need of a White Herb.
Speaking of Omastar's defensive capabilities, it's one of the few Pokemon with access to the entire triad of entry hazards, making a support set possible. Unfortunately, Omastar is a sitting duck when pit against most specially-bulky Pokemon, such as Tentacruel and Kyurem-B, harshly hampering the viability of such an option. Moreover, Omastar's low base Speed will likely leave its team with only a single layer of entry hazards up or none at all. If this isn't enough dissuasion, then be aware that Omastar's third ability, Shell Armor, can be used on a support set in place of Swift Swim or Weak Armor to prevent untimely critical hits. Knock Off might prove useful on a support set as well, but again, Omastar is much better off blasting Pokemon away with its powerful attacks than attempting to support its team. Omastar's options don't stretch too far beyond this point. Omastar has access to oddities such as Wring Out, numerous physical moves, and Seismic Toss, but it's best to veer away from this end of its movepool.
Checks and Counters
Omastar is a rather straightforward Pokemon, but due to its power, it can be difficult to stop if it gets set-up. Resilient special walls, such as Chansey, Slowking, and Ferrothorn, fare well under normal circumstances; however, they must be wary of switching in if multiple layers of entry hazards are present or rain is the current weather condition (or both). Specially defensive Jellicent is a superb answer to Omastar. Jellicent is immune to Hydro Pump, takes a pittance from Ice Beam, can survive a Shell Smash-boosted Hidden Power Grass, and can pound away at Omastar's lower Special Defense stat (although, Jellicent will need Energy Ball or Giga Drain to OHKO Omastar). If Omastar is without a Special Attack boost or sets up early in the game, most special sponges—Celebi in particular due to its Grass STAB—are solid checks to Omastar. Though the list of Pokemon that can outright wall Omastar is short, there are certainly reliable means of stopping it other than countering it.
Keep well in mind that even with the Speed boost provided by Shell Smash or Swift Swim, most Choice Scarf users—anything with base 87 Speed or higher, to be precise—can outspeed and revenge kill Omastar. In addition, Omastar's Rock typing leaves it vulnerable to many common priority attacks, particularly Breloom's Mach Punch; however, Omastar's base 125 Defense means that it must be noticeably weakened before falling to them. If all else fails, prediction can bring Omastar to its knees (tentacles?). Due to its low Speed and weaknesses to common attacking types, anything from a crafty double switch to a carefully timed attack will quickly bring Omastar down.