You cannot apply FGPP to an OU directly.
Fine-grained password policy cannot be applied to an organizational unit (OU) directly. To apply fine-grained password policy to users of an OU, you can use a shadow group.
A shadow group is a global security group that is logically mapped to an OU to enforce a fine-grained password policy. You add users of the OU as members of the newly created shadow group and then apply the fine-grained password policy to this shadow group. You can create additional shadow groups for other OUs as needed. If you move a user from one OU to another, you must update the membership of the corresponding shadow groups.