You have to be an administrator in order to do the following changes therefore the logged on user should be in the Admins group.
Run Active Directory Users and Computers. – > View -> Advanced Features.
Now you have to remove the Deny permissions which preventing the removal of the OU and this has to be done at the OU and the OU parent as follows:
In the OU itself right-click -> Properties -> click the Security tab -> Advanced.->Choose “Everyone group” or SELF in case the same user is trying to delete the OU -> in the permission Entries select Allow Delete All Child Objects -> then Select Object Tab -> clear protect object from accidental delete -> OK
Now at the parent container of the OU right-click -> Properties -> click the Security tab -> In Group or user names, select the Everyone group-> clear the Deny check box for Delete All Child Objects (you don’t need to check the Allow option) -> click OK