Small explanation for tiers in tournaments?

Ok so this is just a small question regarding tiers in competitive/tournament play.

If a tournament states that teams must play in let's say...the OU tier, doesn't that mean that each member of a team must be OU or do the tiers of each Pokémon have to be up to the OU tier?

Example: If a Pokémon team has 5 OU Pokémon and 1 Pokémon in the NeverUsed tier, can it still be used for a tournament played in the OU tier?

I'm really new for getting into this kind of stuff so I just want to learn.

Yup, OU allows everything from OU, UU, RU, NU, PU, and all the BL tiers. Only Ubers and certain moves and abilities are banned (e.g. OHKO moves, evasion moves, Moody, Shadow Tag). Anything that is in the specified tier or under it is allowed.

Thank you!