How to open an OST file

Why an OST will not open

An OST is Outlook’s offline copy of a mailbox that lives on a server: work Exchange, Microsoft 365, Outlook.com, or an IMAP account. Outlook ties the file to the profile that created it, so:

• File › Open › Outlook Data File only accepts PST files, not OST.
• Copying an OST to another computer and pointing Outlook at it does not work.
• Renaming .ost to .pst does not work either. The internal layout is the same family, but Outlook checks.

First: do you need the file at all?

If the account still exists, the mail is on the server. Add the account to Outlook (or sign in at outlook.com or your company’s web mail) and Outlook downloads it again. The OST is only a cache, and a new one is built.

You need the file itself when:

• the account was closed or deleted: an old job, a former school, a removed Microsoft 365 licence;
• the mail was deleted on the server but is still in an old OST;
• you want a local archive of a mailbox that you can keep without the account.

An OST in this situation is called orphaned, and it may be the last copy of that mail.

Find the OST

On Windows, OST files are in:

%LOCALAPPDATA%\Microsoft\Outlook

Paste that into the File Explorer address bar. Files are named after the mailbox, such as [email protected]. Close Outlook before copying one, or Windows reports that the file is in use. Work on the copy, never on the original.

If you are looking in an old backup or a disk from another computer, search it for *.ost. More places to look: where is my PST file.

Turn it into a PST

OST to PST reads the file directly, so it needs neither the account, the server nor Outlook:

1. Open OST to PST and choose the copy of the OST.
2. Preview for free: you see the folders and how many messages will be converted.
3. Convert and save the PST. Every message is read back and compared with the OST.

The conversion runs in your browser, on your computer. The OST is not uploaded.

Then open the PST in classic Outlook (File › Open & Export › Open Outlook Data File), or read it in the PST Viewer without Outlook at all.

What can and cannot come back

• Mail, folders and attachments that Outlook had downloaded come back.
• Attachments Outlook never downloaded cannot be recovered. With some settings Outlook keeps only headers or recent mail offline; what was never in the file is not there to convert.
• Encrypted OST files (Windows file encryption, or files locked to another Windows user) cannot be read without that user’s sign-in.
• The PST is usually bigger than the OST, because Outlook 2013 and later compress OST data and PST files are not compressed.

Frequently asked

Can I just open the OST in the PST Viewer? Yes: the PST Viewer opens OST files too, for reading and saving single messages. Convert to PST when you want the whole mailbox in Outlook.

Is an OST from Outlook 2010 or older supported? Outlook 2013 to Microsoft 365 OST files are supported. Very old ANSI-format files are not.

What is the difference between OST and PST? See PST, OST, MBOX, EML and MSG explained.