To convert OST files to an Office 365 account, use Vartika Software's OST to Office 365 converter tool. This software efficiently migrates emails, contacts, and attachments to Office 365. To convert OST files to PST files, you can use Outlook’s Import/Export Wizard or a third-party conversion tool.
https://www.vartikasoftware.com/product/vs-ost-to-office365-converter.html