Unable to Open an Old OST File After Outlook Profile or Mailbox Changes

Protect the only copy first. Make a byte-for-byte copy of the OST while Outlook is closed and work only with the duplicate. Do not rename it to PST or attach it to a new profile; the formats and mailbox binding are different.

If the original Windows profile still opens the cached mailbox while disconnected, use classic Outlook's File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst) before reconnecting or modifying that profile. Export every folder, then open the PST in a separate Outlook profile and check folder counts, date ranges, attachments, calendar items, and several messages with known content.

For Microsoft 365, the administrator should search the service before treating the OST as the source of record: check soft-deleted users/mailboxes, inactive mailboxes, Online Archive, retention, Recoverable Items, and Microsoft Purview eDiscovery where licensing and authorization permit. If the server copy can be restored, let Outlook build a new OST from it and export any needed archive to PST. An orphaned OST by itself has no Microsoft-supported attach/import path, so preserving the original profile and locating retained server data are the two useful Microsoft routes.