In spite of facing stiff competition from other bulky Water-types, namely Milotic, Blastoise, and Azumarill, Omastar can still hold its own. Omastar is the only Pokemon in UU capable of setting up both Stealth Rock and Spikes, which has turned our little prehistoric friend into a staple on stall teams. Furthermore, Omastar's base 115 Special Attack and access to Swift Swim allows it to function as a fearsome rain sweeper that is capable of plowing through teams in a matter of turns.
Omastar is not without its fair share of faults, however. Its awful defensive typing, which grants it weaknesses to Ground-, Fighting-, Electric-, and Grass-type attacks; low Special Defense; and lack of a reliable recovery move are major drawbacks that severely hinder Omastar’s durability. Despite these disadvantages, do not underestimate Omastar, as it can run (or slide...) circles around unprepared teams.
Although Omastar's typing isn't very auspicious for a defensive Pokemon, one would be foolish to overlook its gargantuan base 125 Defense stat that, when paired with its access to a wide array of support moves, turns Omastar into an amazing asset for defensive teams. The main idea of this set is to switch into something that doesn’t threaten Omastar too much, such as physically-based Flying-, Normal-, or Fire-types, then proceed to set up either Spikes or Stealth Rock as the opponent switches out. If they decide to stay in, Omastar can make use of its impressive Special Attack and STAB Surf to crush most of these threats.
The last moveslot is entirely dependant on personal preference. Ice Beam is the most desirable option, as it deters Grass-types from switching in too readily and deals massive damage to Flying-types, especially Altaria. However, if one doesn't find the aforementioned Pokemon too troublesome, Earth Power is an interesting option that enables Omastar to hit Toxicroak, Qwilfish, and opposing Omastar harder; it especially comes in handy if this set is being run in the lead position, where the latter two are commonly seen. Alternatively, Rest is a viable option that rectifies Omastar’s lack of a recovery move, but should only be used with Heal Bell support.Team Options & Additional Comments >>>
Due to bearing the highest Special Attack of all Water-types in UU and an underwhelming 55 base Speed, Omastar makes a prime candidate for a Rain Dance sweeper. Surf is Omastar's main source of damage, and OHKOes most Pokemon that don't resist it. If more power is desired, Hydro Pump is a good alternative that puts more pressure on bulky special walls such as Clefable and Registeel, even 2HKOing Chansey with Stealth Rock and one layer of Spikes down. Ice Beam allows Omastar to hit Grass-types and Altaria for super effective damage, while Hidden Power Grass is used mainly for Water-types. Because these three moves provide Omastar with amazing coverage (only resisted by Shedinja), Omastar can afford to run a support move in the fourth slot. Rain Dance is the preferred option, as it allows Omastar to act as a self-sustaining sweeper on all sorts of teams while supporting its teammates on dedicated rain teams. Earth Power, despite not achieving much when it comes to coverage, allows Omastar to OHKO Toxicroak, one of the most dangerous threats to rain teams, thus paving the way for one of its teammates, namely Kabutops or Qwilfish, to sweep unhindered.Team Options & Additional Comments >>>
Omastar is blessed as the only UU Pokemon with access to both Stealth Rock and Spikes; coupled with its decent bulk and high Special Attack, this makes Omastar an excellent lead for any UU team in need of entry hazards. This Omastar is built to function as a suicide lead, setting up as many entry hazards as possible before fainting. If an opportunity arises where Omastar can KO the opponent's Pokemon, it should do so; having an immediate advantage against the opposing team while still having a chance to set up entry hazards can be invaluable. Omastar matches up well against almost any common lead, including Uxie and Mesprit without Grass Knot, Registeel, Moltres, Kabutops, Cloyster, other Omastar, and even Rain Dance Electrode, which Omastar can simply outspeed and OHKO with Surf in the rain. With so many favorable match-ups and a Focus Sash, Omastar should be guaranteed either a kill or two layers of entry hazards.
Stealth Rock and Spikes are both key to this set. Putting both moves on Omastar can not only free up an extra slot on your team, but it also gives Omastar a chance to set up multiple layers of hazards if it finds itself in a favorable lead matchup. Omastar's typing, decent Speed, and high defensive stats allow it to set up easily on any Pokemon that is not carrying a Grass-type attack, such as Registeel or Spiritomb (although stronger versions of Shadow Ball Spiritomb can deal serious amounts of damage to Omastar). Surf is a reliable STAB attack that can OHKO Moltres leads and deal significant damage to almost any other lead; even Qwilfish leads take over 40% from Omastar's Surf. The last attack is based on preference, and depends on what Pokemon you would rather have Omastar be able to defeat. Hidden Power Grass can OHKO opposing Omastar that do not run Focus Sash or Rindo Berry, and also ensures that Omastar can outspeed and 2HKO Cloyster with ease. Earth Power allows Omastar to easily handle Toxicroak leads, as well as guaranteeing the 2HKO on Qwilfish.Team Options & Additional Comments >>>
Omastar doesn't have many viable options outside of those that have already been mentioned. Toxic Spikes can be used on the Support set either over Spikes or in the fourth slot, but Spikes tends to be the superior option given the amount of Poison-types in UU that render Toxic Spikes useless, and Omastar becomes severely overloaded if it decides to employ all three entry hazards. Hidden Power Rock gets STAB and can be used on the fourth slot of the Support set as well, but offers inferior coverage when compared to Ice Beam and Earth Power, and doesn't hit anything worthwhile. Omastar also has access to Haze in order to shut down stat boosters, but it is more suited for attacking due to its great Special Attack, and its inability to take on Calm Mind users further diminishes Haze's usefulness on Omastar.
Offensively, Omastar can make use of a Choice Specs set in order to hit common switch-ins harder, most notably 2HKOing Chansey with Hydro Pump 100% of the time if Stealth Rock is down and rain is in play. If one doesn’t want to rely on Rain Dance to boost Omastar's Speed, Rock Polish can be used, which has the advantage of not wearing off after eight turns like rain does. However, without the boost from weather, Omastar will fail to OHKO several threats with Surf and will need to waste a turn boosting its own Speed rather than having it augmented by any other Pokemon that carries Rain Dance.
Checks and Counters
Although it's one of Omastar's greatest allies, Ludicolo also happens to be one of the most reliable counters to the snail. Ludicolo's typing, access to Swift Swim, and superior Speed guarantee that not only will it not take much damage from any of Omastar's attacks, but it also has the option of outspeeding and OHKOing Omastar before Omastar even has a chance to move. Milotic is another great option, as it can shrug off two consecutive super effective Hidden Power Grasses and OHKO Omastar in return with a Hidden Power Grass of its own. Specially defensive Venusaur, despite being unable to take two consecutive Ice Beams, can easily switch into Surf or Hidden Power Grass, take the subsequent Ice Beam, and OHKO Omastar with Energy Ball. Chansey is a complete roadblock for offensive sets, but turns into complete fodder for defensive sets to lay down entry hazards against.