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
Convert orphaned OST files to PST format when your Exchange server is down or decommissioned. Works without an active Exchange connection.
Access and read orphaned OST files when your Exchange server is unavailable. Convert OST to a readable format without needing an active connection.
First conversion free. No credit card required.