In Microsoft's Active Directory, OUs do not confer access permissions, and objects placed within OUs are not automatically assigned access privileges based on their containing OU. Source: Internet
Organizational units do not each have a separate namespace; e.g. user accounts with an identical username (sAMAccountName) in separate OUs within a domain are not allowed, such as "fred. Source: Internet
Only groups are selectable, and members of OUs cannot be collectively assigned rights to directory objects. Source: Internet
Microsoft recommends using OUs rather than domains for structure and to simplify the implementation of policies and administration. Source: Internet
OUs can contain other OUs—domains are containers in this sense. Source: Internet
OUs should be structured primarily to facilitate administrative delegation, and secondarily, to facilitate group policy application. Source: Internet