Omsatar is a fossilized pokemon, evolving from the pokemon Omanyte this pokemon possess godo stats in defense and special attack. It is a rock water pokemon which means it is mega weak to grass type moves.
Omastar is in the Neverused (NU) tier of competitive battling. With the ability to run both defensive and offensive sets, Omastar is a flexible Pokémon in terms of what roles it can fit in, which allows for it to fit on both balanced and offensive teams alike. Its access to all three forms of entry hazards, with Stealth Rock, Spikes, and Toxic Spikes all at its disposal, which allows for it to potentially act as the sole provider of entry hazards for a team. It has good Defense and Special Attack stats, giving it a solid matchup against common forms of entry hazard removal or prevention, such as Xatu, Kabutops, and Pelipper. The best Omastar set is a defensive entry hazards set that takes advantage of Omastar's high base 125 Defense to check certain physical attackers while setting up entry hazards to wear down at opposing teams. It also can run a Shell Smash set that is a very powerful and potent sweeper when it gets the chance to set up, and if backed up by rain, Swift Swim will activate and Omastar becomes a deadly force that becomes very difficult to handle. It can take a majority of priority attacks well due to its high Defense stat while White Herb remedies the Defense drops. Lastly, Omastar can run a more offensive entry hazards set with a Focus Sash to act as a lead for more offensively inclined teams. Omastar is not without flaws, though, as it has a low initial Speed, mediocre Special Defense, and common weaknesses to Grass, Electric, Fighting, and Ground, which can force it out frequently if against offensive Pokémon if it does not have a Shell Smash under its belt.
Omastar is also viable in the Rarelyused (RU) tier of competitive battling. Thanks to a great base 115 Special Attack and a phenomenal boosting move in Shell Smash, Omastar finds itself on teams as a potential late-game cleaner that can overpower opposing teams. It has some good set up opportunities thanks to a high Defense stat and some key resistances, allowing for it to take advantage of the presence of Pokémon like Fletchinder and Drapion. Outside of the Shell Smash set, Omastar can run an effective suicide lead set with a Focus Sash to set up Stealth Rock and Spikes against opposing teams. Omastar's flaws lie in a below average Special Defense stat and low Speed, which can be problematic when trying to set up. It has some unfortunate weaknesses to Fighting, Electric, and Grass, with the latter being especially crippling due to the common presence of Venusaur and Virizion. Its suicide lead set struggles with Blastoise, the tier's most popular Rapid Spin user.
Omastar is a usable option in the Underused (UU) tier of competitive battling. Thanks to Shell Smash, Omastar finds a solid niche despite the omnipresence of Water-types that could give it competition or give it trouble. It has a good enough offensive movepool and high Special Attack stat to work with, which can make it a terrifying sweeper to be matched up against when it gets the chance to set up. It does face competition from Cloyster, though, which may not hit as hard as Omastar but has a superior Speed stat which gives it the ability to outspeed certain Choice Scarf users that Omastar cannot, such as Mienshao and Hydreigon. However, Weak Armor could improve its chances against the likes of Choice Scarf users, as it gives Omastar a speed boost when hit by a physical contact attack in trade for the lowering of its Defense. That does not change the fact that Omastar is more reliant on a Focus Sash than other sweepers due to low Special Defense and Speed stats.
Omastar is also a viable option in the Overused (OU) tier of competitive battling. With Swift Swim, Omastar is an incredibly powerful force under rain, as it can overpower a large chunk of the metagame barring extremely bulky Pokémon like Chansey with a Choice Specs rain-boosted STAB Hydro Pump. Even Chansey is not completely safe from Omastar, as it can have its Eviolite removed through Knock Off. It can act as a great partner for other Swift Swim users like Kingdra, as weakened walls will open up opportunities for the other sweepers to take advantage. It can check Talonflame and Mega Pinsir, which can hit other certain rain sweepers with powerful priority. However, Omastar cannot outspeed certain Choice Scarf users like Landorus-T and Keldeo, and it is heavily reliant on rain to do its job. It has a weakness to Thundurus like a lot of other rain sweepers, as Thundurus can paralyze Omastar with Prankster Thunder Wave, crippling it for the rest of the match. It compounds weaknesses with Kabutops, another rain sweeper with the same typing, giving it synergy issues to work with.