One Team is a whole new squad made from the joined forces of Association members. By loaning their players from their main squad, Managers are building together a new team. 

One Team squad must have 14 players or above in order to participate in the competition.
The competition has 2 phases that repeat every week:
Transfer Phase - which is the time to strategise, loan players and build a strong Association squad
Competition Phase - which consists of three days of One Team matches where you can test out your One Team squad against three more FAs and obtain your hard-earned rewards

Once the One Team squad is formed, the number of players cannot be altered throughout all 4 competitions. This means 4 competitions and 4 weeks of rewards.