PST vs OST Files: What Is the Difference and Can You Open Both?

A PST (Personal Storage Table) file is an open storage container format developed by Microsoft for Outlook. It is used primarily for: - Local email archiving from Exchange or IMAP accounts. - Primary mailbox storage for POP3 email accounts. - Manual exports created via Outlook's Import/Export Wizard.

PST files are portable: you can copy a .pst file to an external drive, move it to another computer, and open it in any desktop Outlook installation via File > Open & Export > Open Outlook Data File.

An OST (Offline Storage Table) file is a local cache copy of a cloud or server mailbox. Outlook creates an OST automatically when configuring accounts using: - Microsoft 365 / Exchange Server. - IMAP email accounts (Outlook 2013 and newer).

The OST file enables you to compose emails, review calendar appointments, and search contacts even when offline. When an internet connection is re-established, Outlook synchronizes changes bidirectionally between the local OST and the remote server.

Unlike PST files, OST files are cryptographically tied to the unique MAPI profile and Exchange mailbox that created them. If: 1. Your email account is deleted or renamed on the server, 2. Your Windows user profile is reinstalled, or 3. You attempt to open the .ost on a different computer,

Microsoft Outlook will refuse to mount the file with the error: "The file cannot be opened. The file is not an Outlook data file (.pst)."

To access emails from an orphaned OST file, you must either parse it with an independent client-side reader or convert it into a standard portable PST archive.

Our free in-browser viewer parses both ANSI/Unicode PST files and cached OST files directly in local device RAM using client-side B-Tree readers. No data leaves your machine, making it safe for inspecting orphaned OST files or archived PST backups without requiring Outlook or Microsoft 365 licenses.