Offline Storage Table — Outlook's local cache of Exchange mailboxes
OST (Offline Storage Table) is Microsoft Outlook's offline cache format for Exchange, Microsoft 365, and IMAP mailboxes. Unlike PST files that are standalone, OST files are synchronized copies of server-side data. When Exchange connectivity is lost, OST files let you continue working offline. OST files cannot be opened directly — they must be converted to another format first.
Origin
Microsoft Outlook + Exchange
Used by
Outlook connected to Exchange Server or Microsoft 365
Convert from or to OST with MailtoPst
3 free methods to convert OST to PST in 2026. Online tool (no install), Outlook archive (if profile works), and desktop software. Includes orphaned OST recovery.
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