To transfer Outlook OST files to an Office 365 account, you'll need to first convert the OST file to PST format using a tool like Outlook Import/Export. Then, use the Office 365 Import Service or a third-party tool to upload the PST file to your Office 365 mailbox.

More info- https://www.vartikasoftware.com/product/vs-ost-to-office365-converter.html