With poor Special Defense, HP, and Speed stats, one might question why Omastar deserves to be used in the Uber realm. The answer is simple: Omastar's ability, Swift Swim, when used in conjunction with Shell Smash, its high Defense stat, and Rock typing, transform it into a fearsome sweeper that cannot be revenged in the rain. With the ability to deal over 80% damage to Ferrothorn with Hydro Pump, it would be unwise to underestimate the fossil, lest you wish to be drowned in the depths of the sea. Do not let this lead you into thinking Omastar is perfect though—its poor base Speed haunts it, as it is easily revenge killed by most Choice Scarf users should the rain provided by Kyogre be taken away by Groudon or Tyranitar. All in all, Omastar is like any other sweeper: support it well, remove its checks, and you will have a monster on your hands.
Omastar's ability combined with a high base 125 Defense stat and its Rock typing allows it to become a very good revenge killer in the rain, capable of revenging even ExtremeSpeed Arceus. Omastar's access to Shell Smash enables it to excel at this role, as it can punish teams that dare try to switch out the Pokemon that is to be revenged, will be promptly met with a monster that OHKOes the metagame after Stealth Rock and two layers of Spikes.
Shell Smash is the crux of the set; it lets Omastar boost its Special Attack stat to sky-high levels, and makes it impossible to revenge under the rain. Hydro Pump provides immense power, allowing Omastar to deal over 80% to Ferrothorn in the rain; however, due to its poor accuracy, one can choose the alternative Surf for more reliable yet less powerful damage. Ice Beam provides fantastic coverage with Surf, hitting Giratina-A for super effective damage, as it is one of the few Pokemon that are not OHKOed by Hydro Pump after Stealth Rock and Spikes. Lastly, Earth Power enables Omastar to hit Dialga for an OHKO, as it also is not OHKOed by Hydro Pump. However, due to the fact this leaves Omastar walled by the likes of Gastrodon and at the mercy of opposing Water-types, it might be more desirable to use Hidden Power Grass to achieve perfect neutral coverage.Team Options & Additional Comments >>>
Omastar can utilize attacks other then those listed in its set, but those attacks do not provide much coverage and are often outclassed. Omastar can utilize AncientPower, as it gets a STAB boost, but sadly, it does not complement Hydro Pump very well, and is fairly weak. You can opt to use Hidden Power Fighting, as it deals a massive amount of damage to Ferrothorn in sun, should the rain Omastar desires be taken away from it. More often then not, however, the moveslot is wasted, as Ferrothorn takes around the same damage from a Hidden Power Fighting as it does from a Hydro Pump in the rain. Omastar can utilize Toxic on a Shell Smash set to hit and OHKO Shedinja switch-ins should your team not be able to provide entry hazard support. Omastar can also take advantage of Swift Swim, and the fact that it receives Stealth Rock and Spikes makes it a decent lead. However, such a set is often outclassed by Qwilfish, which has not only higher Speed, but Taunt, Explosion, Toxic Spikes, and Destiny Bond. Omastar's only advantage over Qwilfish is Stealth Rock, which gives it some viability should you wish to use it.
Checks and Counters
If Omastar smashes, pray to Arceus that it runs Hydro Pump and that its blast of water misses its intended target, as chances are more then likely it will sweep your team. Unfortunately, despite the raw power Omastar brings, it cannot OHKO the whole metagame should it lack entry hazards. If this is the case, Palkia, Ferrothorn, specially defensive Giratina-A, specially defensive Lugia, and bulky Kyogre can all survive an attack and phaze the fossil or OHKO it, destroying its chances to sweep. Shedinja is also a perfect choice if entry hazards aren't up, unless Omastar has AncientPower or Toxic. Omastar is also limited by imperfect coverage: if it lacks Earth Power, then Dialga can also take a hit and revenge the fossil, while if Omastar lacks Hidden Power Grass, Gastrodon can wall it, as Gastrodon will not be OHKOed by any move.
If you still lack any Pokemon to take a hit, then you might as well try to prevent its chances of sweeping your team completely. Scizor and Thundurus can sacrifice themselves to cripple Omastar for the remainder of the match. The former deals a massive amount of damage to—though not necessarily OHKOing—Omastar with Choice Band Bullet Punch, and courtesy of Prankster, the latter is capable of bypassing Omastar's Speed and crippling it with Thunder Wave. Similarly, Whimsicott can cripple Omastar with priority Stun Spore, GrassWhistle, or Encore. Groudon too can make it much easier to revenge kill, as even after a Shell Smash, Omastar's poor Speed drags it down—it is outsped by Choice Scarf base 90 opponents should it lack rain or a second Shell Smash boost. Sadly, Groudon cannot handle the agonizing power of any of the fossil's moves, and will likely fall when replacing rain with sunlight. This all assumes Omastar has already gained a Shell Smash boost though; if you can prevent the monster from smashing by stopping Wobbuffet or a dual screens user, then you will have avoided a crisis.