Hi Osho
As it may seem the OU attribute should be auto populated in AD, depending on the user's immidiate OU, However, that is not the case.
the OU attribute is just another string attribute with no connection to the user's OU in the AD. This attribute just happens to have the name 'ou'.
Most probably it is empty in the Active Directory itself. You can check that in the 'attribute editor' in the Users properties from 'AD Users and Computers Console'
If you want to get the OU for the user you will have to parse the "dn".