OU Tier
OU stands for OverUsed and it is the Standard Metagame that most people play. The OU Tier allows all Pokémon that are in OU and below. All Pokémon that are above the OU Tier are banned. The only Pokémon above OU are Pokémon in the Uber Tier, who are there for being too powerful for OU's metagame. OU is determined by usage so the top used Pokémon are deemed OU.
The List Currently Includes:
Ferrothorn Tyranitar Scizor Gliscor
Latios Excadrill
Reuniclus
Heatran
Rotom (Wash Forme)
Conkeldurr
Jirachi
Politoad
Dragonite
Thundurus
Gengar
Jellicent
Skarmory
Volcarona
Starmie
Gyrados
Hydreigon
Forretress
Blissey
Infernape
Ninetales
Vaporeon
Salamence
Breloom
Chandelure
Scrafty
Tentacruel
Metagross
Terrakion
Swampert
Haxorus
Lucario
Cloyster
Hippowdon
Magnezone
Deoxys (Speed Forme)
Machamp
Landorus
Whimsicott
Latias
Bronzong
Mienshao
Darmanitan
Toxicroak
Virizion
Venusaur
Porygon2
Tornadus
The OU Tier allows anything in the below tiers, it bans anything Uber.
The OU Tier also bans from it's metagame something that is explained in this specific rule:
The Drizzle+Swift Swim CombinationThe Pokémon Politoad gets the ability Drizzle which summons infinite Rain weather. Some other Pokémon get the abilty Swift Swim which doubles their speed in Rain. The combination of using a Pokémon with Drizzle and Swift Swim on the same team is
banned.