Overview – What is OST File Extension?
An Offline Storage File (OST) or Offline Data File format used by Microsoft Outlook to store the Email application data. Offline storage file is created when Microsoft Outlook used with Microsoft Exchange Server.
It also holds all Email messages, notes, journals, tasks, contacts and calendar in a single file.A local file created in a computer in Outlook data file named as Offline storage file. This file used when we work in Cache exchange mode or want to work offline, without connection with Microsoft Exchange Serve
Local OST File is the slave replica of Exchange server Mailbox. File get synchronized when a connection is available.