OAB file, short for Offline Address Book, is associated with Microsoft Exchange Server accounts. OAB file stores clients' names, email-addresses, and office location which can be found in Global Address List (GAL) of Exchange Server. As the name suggests (Offline Address Book), clients can access the OAB file's address list when connection to the MS Exchange is not available i.e. in offline mode.

OAB files needn't to be backed up as the file created & updated by the server automatically. MS Outlook in cached mode updates OAB file every 24 hours.

• Microsoft Exchange Server
• Microsoft Outlook

• The OAB file belongs to the category of Data File (Address Book)

Microsoft Corporation introduced OAB File extension to store the list of the contacts, which can be accessed by Microsoft Outlook.

The MIME type of OAB File is

• application/octet-stream

Different versions of OAB file supports different editions of Exchange Server & Outlook as listed below:

• OAB Version 2 & 3: Supported by Exchange Server 2003, 2007, 2010 and MS Outlook 2003, 2007, 2010
• OAB Version 4: Supported by Exchange Server 2003, 2007, 2010, 2013, 2016 and MS Outlook 2003, 2007, 2010, 2013, 2016

When Outlook clients download OAB file then mutiple files having oab file extension are created which vary according to OAB versions.

• OAB Version 2 & 3:
  • Browse file - Browse.oab / Ubrowse.oab
  • Relative Distinguished Name (RDN) Index file - Rdndex.oab / Urdndex.oab
  • Ambiguous name resolsution (ANR) Index file - Anrdex.oab / Uanrdex.oab
  • Details file- Details.oab / Udetails.oab
  • Display template files - Tmplts.oab / Utmplts.oab
  • Changes file - Pdndex.oab / Updndex.oab
• OAB Version 4:
  • Full Details file
  • Differential Patch file
  • Display template file

Note: Version 3 of OAB file supports Unicode format (having 'U' prefix).

The default location of Outlook OAB file format vary according to the OS installed:

• \Users\user\AppData\Local\Microsoft\Outlook

• drive:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook

OAB file cannot be imported and open within outlook as an individual file. The user requires a Tool to open .oab extensions and view its details. A Freeware is an easily available option to view these files.