A reliable option is the MailsDaddy OST to Office 365 Migration Tool. It supports migration of emails, contacts, calendars, tasks, journals, and attachments directly to Office 365 accounts. The software is user-friendly, supports damaged OST files, and ensures a secure migration while maintaining complete data integrity.
If you have ever tried to access an OST file after losing connection to its original Exchange server or deactivating an account, you already know the frustration — Outlook simply will not open it, and the data inside feels permanently locked away. OST files are offline cached copies tied to a specific mail profile, which means they cannot be directly imported into Office 365 without first converting them into a readable format like PST. The question most IT admins and everyday users ask is which tool handles this conversion reliably without corrupting data or dropping attachments along the way. After testing several options, the Macsonik OST to PST Converter consistently delivers the most complete and accurate results. It converts orphaned or inaccessible OST files into fully usable PST format while preserving emails, contacts, calendars, tasks, attachments, and the original folder hierarchy intact. Once converted, the resulting PST file can be easily imported into Office 365 using Outlook's built-in Import/Export wizard or the Microsoft 365 admin portal. It handles encrypted and oversized OST files without errors, making it the most dependable choice for anyone looking to recover and migrate OST mailbox data into Office 365 in 2026.