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OST (Offline Storage Table) is Outlook's local cache file for Exchange Server and Microsoft 365 mailboxes. When you set up an Exchange account in Outlook, it creates an OST file that mirrors your server-side mailbox — emails, calendar, contacts, everything. You can read, compose, and organise email while offline, and Outlook syncs changes when connectivity returns.
Structurally, OST files use the same internal format as PST files — the same NDB/LTP/Messaging layer architecture. The critical difference is that OST files are tied to the Outlook profile and Exchange account that created them. You can't open someone else's OST file, and you can't import an OST into a different Outlook profile. If you need to move mailbox data, you export to PST first.
OST files can grow large — 50 GB or more for heavy email users — and they're one of the most common causes of Outlook performance issues. A corrupted OST file is also a frequent support call, though the fix is simple: delete it and let Outlook re-sync from the server. Unlike PST corruption, losing an OST means nothing is permanently lost because the server has the canonical copy.