PubRescue What is an .ost file?
An .ost file is the offline copy of a mailbox that classic Outlook for Windows keeps on the computer when it is connected to an Exchange, Microsoft 365 or Outlook.com account. Outlook uses it so you can read mail without a connection, and it syncs it with the server.
Outlook only opens an .ost file together with the account it belongs to. If that account was deleted, the password no longer works, or you moved to a new PC, Outlook won't open the file on its own. That's usually when people go looking for an OST viewer.
When this helps
- The account is gone. A mailbox was removed, a subscription ended, or a business closed, and the .ost on an old PC is the only copy left.
- An old computer. You copied the files off a laptop you're retiring and want to check what's in the mail before you wipe it.
- No Outlook. You're on a Mac, a Chromebook or a PC without Outlook and just need to read some messages.
Mail from a work or school account belongs to that organization. If the file came from an employer, check that you're allowed to keep and read it.
Where to find the .ost file
On Windows, classic Outlook keeps .ost files in C:\Users\<your name>\AppData\Local\Microsoft\Outlook. The file is usually named after the email address. AppData is a hidden folder: type %localappdata%\Microsoft\Outlook into the File Explorer address bar to go there. Close Outlook before you copy the file. The new Outlook for Windows app stores its offline mail differently and doesn't create these files.
Open an .ost file, step by step
- Drag the .ost file onto the box at the top of this page, or click Choose files.
- Pick a folder in the Folder drop-down. Folders are indented like the tree in Outlook and show how many messages they hold.
- Read and search. Messages are listed newest first. The search box filters by subject, sender and recipients; tick Search message text too to look inside the messages.
- Save what you need. Download attachments, or tick messages (or use Select all) and choose Save selected as PDF, Export selected as .eml (ZIP) or Export as MBOX.
Big files are fine. The viewer reads only the parts it needs, so a multi-gigabyte .ost opens quickly without being uploaded.
Can it convert OST to PST?
No. The viewer doesn't write .pst files. To move the mail into another program, export each folder as MBOX: Apple Mail imports MBOX directly (File > Import Mailboxes), and Thunderbird does with the ImportExportTools NG add-on. Exported .eml files open in almost any mail program. For records, PDF is often the simplest: up to 500 messages per PDF, each starting on a new page.
Your email stays on your computer
The viewer runs entirely in your browser. Your .ost file is never uploaded to our server and never stored. Only this page and its scripts come from us; reading the file and every export happen on your own computer.
Limits
- .ost files from Outlook 2013 and later are supported, but files from some setups may not open.
- Calendar items, contacts, tasks and notes are listed with a label and open as simple items.
- Encrypted (S/MIME) messages aren't decrypted.
- Damaged parts of a corrupt file are skipped, and the viewer tells you how many.