Reactive Training Systems

To import an Outlook OST file into an Office 365 account, first convert the OST file to PST using tools like Outlook's export feature or third-party software. Once converted, use Office 365's import feature to upload the PST. To import bulk OST files to an Office 365 account, convert OST to PST, then use Office 365 Import Service.

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