Unable to import an OST file to Office 365

Trying to load a departed employee's OST into our Office 365 tenant. The Import/Export wizard will not take it and the compliance center's network upload rejects it too.

Every import door seems closed to OST specifically. Is that by design?

Entirely by design: every import mechanism Microsoft ships, the desktop wizard, the compliance center's network upload, the drive shipping service, speaks PST exclusively. OST was never meant to travel, it is a cache bound to its dead profile, so the doors are not broken, they were simply never built for that file. The move is one conversion, then any door you like.

1

Convert the OST to PST with a standalone converter that reads the file without its original profile, SysTools OST Converter does this directly, producing an ordinary PST with the folder structure intact.

2

Pick the import door by scale. For one mailbox the manual route is quickest: give yourself or the target user the account in desktop Outlook, attach the PST via Open Outlook Data File and drag folders into the 365 mailbox, they upload as they land.

3

For a formal or bulk import use the compliance center: Data lifecycle management, Import, create a PST import job, upload the file with the AzCopy command it hands you and map it to the target mailbox in the CSV.

4

Verify the item counts against the converted PST when the import completes, then archive the original OST and the PST somewhere cold in case legal ever asks.

Import target worth considering while you are at it: an inactive mailbox or a shared mailbox often suits departed employee data better than a live user's mailbox, keeping the inherited mail separate and searchable without a license burning on it. The compliance import maps to those targets exactly the same way.

Converted and imported into a shared mailbox through the compliance job, counts match. The by design explanation ended an afternoon of fighting the wrong door.