An OST (Offline Storage Table) file is tied to an Outlook profile and Exchange account, which makes direct migration to Office 365 (Microsoft 365 Cloud) a challenge. Since Microsoft does not provide a native method to directly upload OST files, the best solution is to use a dedicated third-party migration tool such as the ATS OST to PST Converter with Office 365 support.

This software is engineered to recover data from healthy, corrupted, or orphaned OST files and migrate it directly to Office 365 mailboxes without requiring Outlook or Exchange connectivity. It preserves all critical mailbox items such as emails, attachments, contacts, calendars, notes, and tasks while maintaining folder hierarchy and metadata (To, From, Cc, Bcc, Subject, Date & Time).

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