Outlook Offline Data File
Generic / Ambiguous DataOST is Outlook’s offline synchronized mailbox data, not a portable substitute for PST. Check its original account and configuration, especially unsynchronized items and differences in cached date ranges.
- What it is used for
- Offline synchronized mailbox data
- What the file contains
- Binary data
All sections, questions and sources
Content reviewed： · Editor：Kai
Opening steps
Choose your environment or purpose to see its steps.
Menu labels and support vary by app, OS and version. Preserve the original before making changes.
Troubleshooting by symptom
- Cannot open standalone: check account linkage; do not import it as PST.
- Older mail absent: compare cached time ranges with server-side items.
- Sync errors: check connectivity/account settings and unsynchronized items before considering recreation.
- Server items absent locally: inspect cache period/offline scope.
- Outbox retains items: protect unsent/unsynchronized work before recreation.
- Copied OST cannot be added: inspect original-account/profile dependencies.
Original Outlook environment → supported export
- Check synchronization in the original account and protect unsynchronized data.
- Use that Outlook version’s supported export and verify folders, date scope and item counts.
- Inspect supported exports for date coverage, counts, attachments and required items; do not treat this as guaranteed orphan recovery.
Renaming OST to PST is not conversion. Recovering an orphaned OST differs from routine migration and may be incomplete.
Cache versus backup
OST stores offline synchronized content; its scope/status may omit parts of the mailbox.
Export needed data while the original environment is accessible; a copied cache is not necessarily a complete backup.
Source for these steps：Microsoft · Create an Outlook Data File (.pst) to save your information / Microsoft · How to work offline in Outlook for Windows / Microsoft · Import email, contacts, and calendar from an Outlook .pst file
Inspect unsynchronized items before recreation
Resynchronization restores available server data within scope; inspect unsent/unsynced changes and needed local-only data first.
Orphan recovery is not a rename-based PST migration; investigate account/backups and validate recovered counts/content.
Source for these steps：Microsoft · Create an Outlook Data File (.pst) to save your information / Microsoft · How to work offline in Outlook for Windows / Microsoft · Import email, contacts, and calendar from an Outlook .pst file
Related-format differences
| Format | Main role | What to check |
|---|---|---|
| OST | Synchronization cache | Account/synchronization scope |
| PST | Independent storage/migration | Inspect export coverage |
| Server mailbox | Server-side data | Distinct from unsynchronized local items |
Handling and security
Protect account/private mail data. Do not delete before checking unsynchronized items; review data handling before sharing with recovery services.
Questions and answers
Can I copy it to another PC and open it?
It is not a standalone portable archive like PST.
Will deleting it always restore everything?
Check for unsynchronized items first.
Is it a complete mail backup?
Cache periods/synchronization scope may omit items.
Is OST always identical to the server?
It depends on cache scope and sync completion; inspect unsynchronized data and date coverage.
What should I do before removing the original account?
Export and verify needed items through supported methods and check for unsynchronized work first.
Format facts: name, steward and MIME type
- Formal name
- Outlook Offline Storage Table
- Steward / specification
- Microsoft
- MIME type
- Depends on the producer or server; an extension alone does not establish a MIME type.
Primary and official sources
- Software documentationCreate an Outlook Data File (.pst) to save your information
Distinguishes PST storage and OST offline data.
Microsoft · Link checked： - Software documentationHow to work offline in Outlook for Windows
Explains offline synchronization and connection state.
Microsoft · Link checked： - Software documentationImport email, contacts, and calendar from an Outlook .pst file
Documents import, duplicates and application requirements.
Microsoft · Link checked：