Offline Storage Table (OST)
An OST file, or offline storage table (.ost) file, is an offline Outlook Data File used by Microsoft Exchange Server that enables users to work with their messages and mail data even when access to the mail server isn't available. OST files are used in two scenarios: with Microsoft Exchange Server’s Cached Exchange Mode and with the Outlook Connector for Windows Live Hotmail when accessing Hotmail mail.
While a Microsoft Exchange Server user’s e-mails and other data items are normally saved directly on the mail server, OST files are an available option that enable a local copy of all the data to be stored on the user’s computer. These items are then automatically synched when access to the mail server is restored.
One of the biggest differences between OST files and the personal storage table (.pst) files used by Microsoft Outlook for managing POP3, IMAP and web-based mail accounts is that unlike PST files, OST files are tied to the mail account configuration and cannot be opened or backed up individually.