In order to be eligible for the One Team competition rewards each week, a Manager must loan players to the FA squad before the first Transfer Phase ends and the competition kicks off. 
Once the One Team squad qualifies for the competition, with 14 players or more, the number of players of the joined team cannot be altered.

The exclusive competition requires commitment; the number of One Team players cannot be changed throughout the season. This means that players participating in the One Team competition will not be available for any official matches. However, you can bring back a player from the One Team squad during every Transfer Phase by swapping it with another player from your main squad.
Note: The swaps are limited per Transfer Phase.

As soon as the fourth and last One Team competition of the season ends, all players return to their main squads.