Open OST File · OST Viewer Without Outlook
An OST is the offline file where Outlook keeps a local copy of Exchange or Microsoft 365 mail. Normally only the Outlook that created it, on that computer, can open it, so once the account is removed or the computer is replaced there is no obvious way in. Drop the OST here to read its folders and emails in your browser. The file never leaves your device.
No upload
Drop your files here
Drop .eml, .msg, .pst, .ost, .mbox or .emlx files, a folder or a ZIP to browse them like a mailbox. Files open only inside this browser.
How to open an OST file
- 1
Find the .ost file and drop it
On Windows it usually sits under AppData\Local\Microsoft\Outlook in your user folder. Copy it out and drop it on this page.
- 2
Look through the folder tree
Folders such as Inbox and Sent Items appear first. Pick a folder for its emails, then an email for its body and attachments.
- 3
Take out what you need
Save emails as PDF or .eml, and attachments one by one or as a ZIP.
About OST files
Normally only that computer's Outlook opens an OST
When Outlook is connected to Exchange, Microsoft 365 or some IMAP accounts, the mail lives on the server and Outlook keeps a copy on the computer as an .ost file. It is a cache that lets you read mail while offline.
That file is tied to the Outlook profile on the computer that made it. Another Outlook cannot open it with "Open", and it cannot be imported the way a PST can. That is why people get stuck when all that is left is the OST: after the account was deleted, after leaving a job, or after Outlook stopped working.
| PST | OST | |
|---|---|---|
| Created by | Export, Archive, POP3 accounts | Offline copy of Exchange, Microsoft 365 and similar |
| In another Outlook | Can be opened or imported | Cannot be opened |
| In this viewer | Opens | Opens (with some exceptions, below) |
Some OST files will not open
The internal layout of OST files varies between Outlook versions. This viewer reads the common layouts, but some newer-format OST files and damaged files may not open. When that happens, it says so.
If the account still exists, reading the mail in Outlook or webmail is the surest route. When only the OST is left, try this viewer first.
Large OST files stay light
Because an OST mirrors a whole mailbox, it is often bigger than a PST. The file is never loaded into memory at once; only the needed parts are read, so the folder tree appears first and each email loads when you pick it. There is no limit on the number of emails.
Common questions
Can I open an OST copied from another computer?+
Usually, yes. The viewer reads the file itself, regardless of any Outlook profile. Some newer-format or damaged OST files may not open.
Where is my OST file?+
On Windows it is usually under C:\Users\your-name\AppData\Local\Microsoft\Outlook. AppData is hidden, so type %localappdata%\Microsoft\Outlook into the File Explorer address bar to jump there.
Does Outlook need to be closed?+
An OST that Outlook is using is locked and may not be selectable. Close Outlook, or copy the file somewhere else, then drop the copy.
Is the OST sent to a server?+
No. The file never leaves your device and is read inside this browser.
Can I convert OST to PST?+
There is no PST conversion. You can take emails out as .eml or PDF, attachments as a ZIP and the list as CSV.
There are a lot of emails. Is that a problem?+
No. There is no limit on the number of emails, and the list is drawn in pieces so tens of thousands still scroll.
What about PST files?+
The "Open PST file" page opens them in the same viewer. A password-protected PST is not opened, and the viewer tells you so.