Apple Open Directory, an LDAP-compatible directory service for Mac OS X Server
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The Open Directory Project (ODP), a human-maintained directory of websites also known as DMOZ
Australian Open | French Open | Open University | open source | U.S. Open | Wacken Open Air | French Directory | The Open Championship | U.S. Open (tennis) | All England Open Badminton Championships | U.S. Open (golf) | Open Source | Open-pit mining | 2006 Australian Open | Open Software Foundation | Indira Gandhi National Open University | Lamar Hunt U.S. Open Cup | Active Directory | Lightweight Directory Access Protocol | 2005 Australian Open – Men's Singles | U.S. Women's Open | open access | Gerry Weber Open | Ulrich's Periodicals Directory | Swedish Open | International GT Open | Federal Open Market Committee | 2014 Australian Open – Men's Singles | 2008 Australian Open – Men's Singles#Finals | WTA German Open |
Any Mac OS X Server system prior to 10.7 (Lion) configured as an Open Directory Master can act as a Windows Primary Domain Controller (PDC), providing domain authentication services to Microsoft Windows clients.