Planning to Migrate OST Files to Microsoft 365 - Need Some Guidance Before I Start

Hi,

A few thoughts based on what's worked in similar cases.

For orphaned OST files, Outlook can't help since there's no active profile to sync against. You'll need a tool that can open the OST directly as a standalone file and export it to PST, which is what Office 365's Network Upload / PST Import service expects.

For corrupted ones, be careful. Some tools silently drop items during conversion without warning you, which is worse than an outright failure. I'd run a repair pass on the corruption first, then migrate from the repaired file rather than the original. I've used EdbMails OST to PST Converter for similar cases, it can open orphaned OSTs without the original profile and has a built-in repair step before export. Worth testing on one file first before running the whole batch.

For verifying integrity afterward, compare item counts per folder rather than eyeballing it, and check calendars, contacts, and nested folders specifically since those usually break first. For a large batch, test a few mailboxes fully before touching production.

Also always work from a copy, never the original OSTs, especially the corrupted ones.