Since Outlook verifies the file against the associated account, it cannot normally open an orphaned OST. In such cases, a standalone solution like the SysInfo OST Viewer Tool can read the OST file directly without requiring Outlook, Exchange, or an active Microsoft 365 account. You can view emails, contacts, calendars, notes, tasks, and attachments, and if needed, export the data into formats such as PST, EML, PDF, MSG, MBOX, or CSV. More: https://www.sysinfotools.com/recovery/ost-file-viewer.php