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.