>>30815871If a Pokemon's tier is UU or lower, you can use it in OU.
If a Pokemon's tier is OU, you cannot use it in UU or lower.
It works this way for every tier. You can bring lowered tiered Pokemon up, but you cannot bring higher tier Pokemon down. This only applies if you're playing on Showdown or as an agreement with somebody you're playing with.
>>30815972You can do that if the tier you're playing in is OU.