What is a OAB file? An OAB file is an offline address book created by Microsoft Outlook to store contact information for offline access when disconnected from Exchange Online. It contains a directory of names, email addresses, and other contact details for users within an organization synced from Exchange Online. OAB files allow users to view and access their address book without an active Internet connection, making it helpful to access contacts offline or in areas with limited to no connectivity.

More Information Microsoft Outlook downloads the Offline Address Book periodically (typically every 8 hours) to ensure it stays updated with the latest contact information from the Exchange server. The local OAB file copy helps reduce network usage by limiting the need to fetch contact details from the server each time they are accessed. The file type also supports efficient email communication within large organizations by offering quick, offline access to frequently needed contact information. NOTE: You can download an offline address book (OAB file) from Outlook by selecting File → Account Settings → Download Address Book .