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.

The best method would be to extract the data from the original mailbox. If its been a while and the mailbox is no longer available in soft deleted or inactive state. You may run a eDiscovery tenant wide with the person in participants, this will pull together any email that this person was around using the existing mailboxes as multiple sources to consolidate almost a good picture of the original mailbox content without raising any privacy concerns.

Answers your questions, which is mostly not supported by MS:

• Is there any Microsoft-recommended method to access an orphaned OST file? - No
• What checks should be performed before trying to recover data from the OST? - Original PC with the profile intact, can be tried loading in Offline mode
• How do administrators usually handle old OST files when the original mailbox is no longer available? - OSTs are not meant to be access offline without the actual profiles intact, its just a cache for live\temp usage only.
• What is the best way to validate recovered mailbox data? - N/A. There isn't much to validate, if you don't have any reference to match with, just check the items for To and From

If the original Microsoft 365 or Exchange mailbox still exists, create a new Outlook profile and let Outlook synchronize the mailbox again. An old OST file generally cannot simply be attached to another Outlook profile.

If the original mailbox is no longer available, keep a backup copy of the OST and first check whether the required data exists in a PST, backup, or retention location.

If the OST is the only remaining source, a third-party recovery tool may be required. EdbMails OST to PST Converter can scan an accessible orphaned or damaged OST, preview recoverable mailbox items, and export the readable data to PST. Recovery depends on the condition of the source OST.

After export, open the PST in Outlook and verify the folder structure, emails, attachments, dates, and important mailbox items.

You can use a 3rd party conversion tool

There is no direct way to open orphaned OST files once the original mailbox or Outlook profile is gone.You may try to reconnect to the original mailbox if it still exists; otherwise, administrators must convert the OST to PST using third‑party tools.

https://support.microsoft.com/en-us/outlook/open-or-import-items-from-an-offline-outlook-data-file-ost