How to Open OST File: View Emails or Convert OST to PST
An OST file is Outlook's offline copy of an Exchange, Microsoft 365 or Outlook.com mailbox. Outlook won't open it with "Open Outlook Data File", because it belongs to one account profile. If the account still works, the mail is safe on the server. If it doesn't (old job, deleted account, broken computer), the free viewer below opens the OST in your browser: browse folders, read messages, download attachments and save emails as .eml, without uploading anything.
Choose what you need
Open OST File Online: Read Emails Without Outlook (Free)
Drop an .ost file below. The viewer reads the Outlook offline cache directly, including the compressed 4 KB format used by Outlook 2013 and later, and shows its folders, messages and attachments. Open any message to read it, download its attachments or save it as .eml.
Good to know: the OST is read on your device and never uploaded. Copy it while Outlook is closed, because Outlook keeps it locked while running.
🔒 Nothing is uploaded — your mailbox never leaves this deviceOn this page
What is an OST file?
An OST (Offline Storage Table) file is a local, synchronized copy of a mailbox that lives on a server: Exchange, Microsoft 365, Outlook.com or IMAP accounts in recent Outlook versions. Outlook keeps it so you can read mail offline and work faster. It uses the same database structure as a PST, and Outlook 2013 and later use a newer 4 KB-page variant with compressed data.
| OST | PST | |
|---|---|---|
| Purpose | Offline cache of a server mailbox | Archive, backup or POP mailbox |
| Master copy | On the server | The file itself |
| Open in Outlook | Only through its account profile | File > Open Outlook Data File |
| Safe to delete? | Yes, Outlook rebuilds it (if the account works) | No, it may be the only copy |
How to open an OST file: quick answer
| Situation | What to do |
|---|---|
| The account still works | Sign in to Outlook or Outlook on the web: everything is on the server |
| Account gone, OST left over | Open it with the viewer above and save what you need |
| Need a PST copy | Export from Outlook while the account works (see below) |
| Outlook reports OST errors | Rebuild the OST by letting Outlook re-sync |
Why Outlook won't open an OST directly
An OST is tied to the Outlook profile and account that created it. Choosing File > Open & Export > Open Outlook Data File and selecting an .ost gives an error, and copying an OST to another computer doesn't make it usable there. Outlook only reads it as the cache of its own account.
So the right "fix" depends on whether the mailbox still exists. If it does, add the account to Outlook on any computer and the mail downloads again from the server. If the account is closed or you've lost access, the OST may be the only copy left, and a viewer that reads the file directly, like the one on this page, is the way in.
Only open mailboxes that belong to you or that you're authorized to access. Company OST files usually belong to the employer.
Convert OST to PST
While the account still works (recommended)
- In classic Outlook choose File > Open & Export > Import/Export.
- Select Export to a file > Outlook Data File (.pst).
- Pick the mailbox (or a folder), tick Include subfolders, choose where to save the .pst and finish.
When the account no longer works
Outlook can't export from an orphaned OST. Open it with the viewer above and save the messages you need as .eml files, which any mail app can import, and download the attachments. Commercial OST-to-PST converters exist for converting very large mailboxes in one go.
Where Outlook keeps the OST
On Windows the OST is usually in C:\Users\<you>\AppData\Local\Microsoft\Outlook. Type %localappdata%\Microsoft\Outlook in the File Explorer address bar to go there. The file is named after the email address, for example name@company.com.ost. In Outlook you can see its path in File > Account Settings > Account Settings > Data Files.
Outlook keeps the file open while it runs; close Outlook before copying it.
OST problems, errors and rebuilding
"Outlook data file cannot be accessed" or sync errors
If the account works, the simplest fix is to rebuild the OST: close Outlook, rename the .ost (for example to .old) and start Outlook again. It downloads a fresh copy from the server. Keep the old file until you've checked that everything came back.
Scanpst on an OST
Microsoft's Inbox Repair Tool (scanpst.exe) can check OST files too, but since the server holds the master copy, rebuilding is usually faster and safer.
The OST is very large
Outlook caches everything by default. In the account settings you can limit "Download email for the past" to a shorter period; older mail stays on the server.
The viewer shows fewer items than Outlook
Items that were still downloading, or stored only on the server, aren't in the OST. Shared mailboxes and online archives may also be missing.
Sources and technical references
- Repair Outlook Data Files (.pst and .ost) — Microsoft Support
- Open and find items in an Outlook Data File (.pst) — Microsoft Support
- [MS-PST]: Outlook Personal Folders (.pst) File Format — Microsoft Learn
- Personal Storage Table (PST and OST) — Wikipedia
Frequently asked questions
How do I open an OST file?
If the email account still works, just sign in to Outlook or Outlook on the web: the mail is on the server. To read an orphaned OST, drop it into the viewer on this page, which shows its folders and messages in your browser.
Why can't Outlook open my OST file?
An OST belongs to the Outlook profile and account that created it, so Open Outlook Data File doesn't accept it and it won't work on another computer. Use the account itself, or a viewer that reads the file directly.
How do I convert OST to PST?
While the account works, use classic Outlook's File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst). For an orphaned OST, open it with the viewer here and save messages as .eml, or use a dedicated converter.
Where is the OST file stored?
Usually in C:\Users\<you>\AppData\Local\Microsoft\Outlook, named after the email address. Type %localappdata%\Microsoft\Outlook in File Explorer, or check File > Account Settings > Data Files in Outlook.
Can I delete an OST file?
If the account still works, yes: close Outlook and delete or rename it, and Outlook downloads a fresh copy. If the account no longer exists, the OST may be the only copy of the mail, so keep it.
How do I fix OST synchronization errors?
Rebuild the OST: close Outlook, rename the .ost file and start Outlook again so it re-downloads the mailbox. Keep the old file until you've checked that everything synced.
Can I open an OST file on a Mac?
Outlook for Mac doesn't use OST files. Open the OST with the browser viewer on this page on any computer and save the messages you need as .eml, which Apple Mail opens.
Is it legal to open someone's OST file?
Only open mailboxes that are yours or that you're authorized to access. A company OST usually belongs to the employer, even if it's on your work computer.
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