The short answer
- OST is an Outlook offline data file that stores a local copy of Exchange mailbox items for offline work and synchronization, with access tied to the associated Outlook account.
- On Windows, open Outlook with the account associated with the OST and access the mailbox there. Microsoft's documented recovery route is to recreate the OST and download the mailbox again. A standalone OST should not be treated as a portable PST import file. Microsoft does not document a way to open a standalone OST on a Mac or phone; the OST is only a local cache, while the mailbox data remains in the Exchange mailbox.
- Protect an OST as sensitive mailbox data: MITRE identifies local Outlook caches, including OST files, as targets for email collection. Attachments also need caution; Microsoft describes them as a common route for viruses and blocks certain types such as EXE. The extension alone does not establish safety or show that an OST itself executes code.
OST files: opening, recovery and PST export
An OST can contain email, appointments, contacts, tasks, journal entries, notes, posted items and documents. It participates in synchronization between the local Outlook data and the Exchange mailbox. Adding an account that uses OST makes Outlook create the file and download server data.
If the associated mailbox remains accessible in Outlook, export the required data to PST. Before recreating an OST, export items that exist only locally, then download server data and import the saved PST data. Attachments are retained, but folder views, permissions, AutoArchive settings, message rules and blocked senders lists are not exported.
How to open it
On Windows, open Outlook with the account associated with the OST and access the mailbox there. Microsoft's documented recovery route is to recreate the OST and download the mailbox again. A standalone OST should not be treated as a portable PST import file. Microsoft does not document a way to open a standalone OST on a Mac or phone; the OST is only a local cache, while the mailbox data remains in the Exchange mailbox.
On Windows, open Outlook with the account associated with the OST and access the mailbox there. Microsoft's documented recovery route is to recreate the OST and download the mailbox again. A standalone OST should not be treated as a portable PST import file. Microsoft does not document a way to open a standalone OST on a Mac or phone; the OST is only a local cache, while the mailbox data remains in the Exchange mailbox.
If the associated mailbox remains accessible in Outlook, export the required data to PST. Before recreating an OST, export items that exist only locally, then download server data and import the saved PST data. Attachments are retained, but folder views, permissions, AutoArchive settings, message rules and blocked senders lists are not exported.
File safety
Protect an OST as sensitive mailbox data: MITRE identifies local Outlook caches, including OST files, as targets for email collection. Attachments also need caution; Microsoft describes them as a common route for viruses and blocks certain types such as EXE. The extension alone does not establish safety or show that an OST itself executes code.
An unknown sender, a name ending in two extensions, or contents that do not match the extension — check before you open it.
Converting it
If the associated mailbox remains accessible in Outlook, export the required data to PST. Before recreating an OST, export items that exist only locally, then download server data and import the saved PST data. Attachments are retained, but folder views, permissions, AutoArchive settings, message rules and blocked senders lists are not exported.
Convert a copy, never the original, then compare quality, metadata, and layout against what you started with.
Common questions
What is an OST file used for?
An OST stores local copies of Exchange mailbox items so Outlook can support offline work and synchronize with the server. Its contents can include messages, contacts, appointments, tasks and notes.
How do I open an OST file on Windows?
Open Outlook using the account associated with the OST and access its mailbox. If the cache needs replacing, Microsoft's documented route is to recreate the OST and synchronize server data again, after preserving local-only items.
Can I read an OST file without the original account?
Microsoft does not document a way to open or import an OST on its own, detached from its account. If the Exchange mailbox still exists, add or re-add that account in Outlook to create a new OST and download the server data again. Items that never reached the server cannot be recovered this way.
How do I repair a corrupted OST file that will not open?
Corruption can cause opening failures, synchronization errors or missing items. Microsoft describes checking and repairing internal structures and recreating the OST. Preserve accessible local-only data in PST before recreation.
Can I recreate an OST without losing local email?
First export items that exist only locally to PST. Recreate the OST, let Outlook download the server mailbox, then import the saved PST data. Server synchronization alone cannot restore items that never reached the server.
How do I convert an OST file to PST?
When the associated mailbox is accessible in Outlook, export the required mailbox data to PST. This is an Outlook export route; Microsoft does not document a conversion method for an OST detached from its account.
What is the difference between OST and PST, and which should I use?
OST is the synchronized offline copy associated with an Outlook account. PST is the export and import file used in the documented migration route. Use the OST through its account for offline work and PST for exporting transferable mailbox data.
Which settings are lost when OST mailbox data is exported to PST?
Attachments are retained, but folder views, permissions, AutoArchive settings, message rules and blocked senders lists are not exported. A successful PST export therefore does not preserve every mailbox setting.
What are the OST attachment security risks?
Attachments inside cached email may be dangerous. Microsoft identifies attachments as a common virus transmission route and blocks types including EXE. That does not establish that the OST container itself executes code.
Does an OST file contain private email data?
It can contain local email and other mailbox items. MITRE explicitly identifies OST caches as potential sources for attackers collecting sensitive messages. Treat the file as private mailbox data when copying or sharing it.
Related formats
Sources
Open or import items from an offline Outlook Data File (.ost) · Microsoft Support
Synchronization problems occur in Outlook and in Outlook Web App · Microsoft Learn
Import email, contacts, and calendar from an Outlook .pst file · Microsoft Support
Blocked attachments in Outlook · Microsoft Support
Email Collection: Local Email Collection, T1114.001 · MITRE ATT&CK