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How to open an OST file without Outlook

Outlook won't open a standalone .ost file. You can read the messages by converting the file to PST, or by opening it in a reader that understands the format.

What an OST file is

An OST (Offline Storage Table) is the local copy of an Exchange, Microsoft 365, Outlook.com or IMAP mailbox. Outlook creates it to work offline and keeps it in sync with the server.

That is why it is tied to the profile and account that created it. Under File, Open & Export, Open Outlook Data File, Outlook only accepts .pst files; an .ost chosen there is rejected.

First: does the account still exist?

If the account is still active and you have the password, the simplest route is to set it up again in Outlook: the messages come back from the server and the old .ost isn't needed.

Converting makes sense when the account was disabled or deleted, the password is no longer available, the server was shut down, or you want an independent copy that opens on any computer.

Where the .ost file is

On Windows, the default location is %LOCALAPPDATA%\Microsoft\Outlook. Paste that into the address bar of the file picker and press Enter: the folder is hidden and doesn't show up when browsing. The file is usually named after the account's email address, for example you@company.com.ost.

How to read and convert the OST in your browser

1. Close Outlook. While it is running, the file is locked. If you can't close it, copy the .ost to another folder and use the copy.
2. Open the converter and drag the file onto the page.
3. Check the folders and messages. Reading is free and unlimited.
4. Under Convert, build the PST and choose where to save it.
5. In Outlook, open the PST under File, Open & Export, Open Outlook Data File.

The converted PST includes folders, messages, attachments, calendar and contacts, and works without the original account.