In Server 2008, when you create a new OU, by default the Protect container from accidental deletion checkbox if checked. This helps to prevent you, or someone else, from deleting something they should not. The problem comes from what to do if you really do need to delete it. Follow this procedure to undue that setting.
• Enable Advanced Features in Active Directory Users and computers.
• Open the OU’s properties
• Click the Security tab.
• Click the Advanced button.
• Click the entry that starts as Deny Everyone.
• Click Edit.
• In the Deny column, remove the checks for Delete and Delete Subtree.
• Click OK 3 times.
You can now delete the OU.