You want to Import OST files into Outlook for Microsoft 365. New users can try out the free method to import their OST files into Outlook. The manual method is converting your One OST files into PST file format. But users know about the technical knowledge. Users can convert the OST files by manual process and follow the step-by-step. Users can make mistakes and lose all their data and not recover it. The manual method is a very long and time-consuming process. I suggest the third-party application try out. Many applications are available on Google.