Password policies are applied only at domain level and not at OU Level. You may create a child domain and apply your password policy.
Alternate method in Win2K8 R2 is go for Fine-grained password policy. Create a shadow group (Shadow group is a global security group that is logically mapped to an OU to enforce a fine-grained password policy). Add users of the OU as the members of a shadow group and then apply the fine-grained password policy to this shadow group.
ThanksMANU PHILIP Microsoft MVP(Exchange Server)| MCITP | MCTS | MCSA
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