I found a file called something.ost in my Outlook folder and a different file called archive.pst. I do not understand the difference. Can I copy an OST file to another computer and open it there?
What is an OST file and how is it different from a PST file?
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Victor Gibson November 8, 2026
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Both are Outlook data files, but they do different jobs.
An OST (offline storage table) is a local cache of a mailbox that lives on a server, like Exchange, Microsoft 365 or an IMAP account. Outlook uses it so you can work without a connection. The real mail is on the server.
A PST (personal storage table) is a real container for mail. When mail is in a PST, that file is the only copy unless you back it up. Archives and exports are usually PST files.
To answer your question, an OST is tied to the mailbox and profile that created it. You can not simply copy it to another PC and open it. Add the account on the new PC and Outlook builds a fresh OST for you.
So if my Exchange account is deleted, the OST is useless?
Mostly yes. It still holds a copy of the data but Outlook will not open it without the account it belongs to. If you need that mail, export it to a PST while the account still exists, using File > Open & Export > Import/Export. After the account is gone that is no longer possible in Outlook.