You can move your mailboxes and folders from OST files to Office 365 with the help of OST converter software. That is, EdbMails' OST to PST converter and import tool can import OST files directly to Office 365 tenants and Exchange servers without the need for Outlook. Its feature for incremental migration makes sure that the data is always the same by preventing duplicates during later migrations.

https://www.edbmails.com/pages/ost-to-pst-converter.html
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