To migrate OST mailbox data to Office 365, you first need to access the contents of the OST file and then transfer them to your Office 365 mailbox. One of the easiest ways to do this is by using the SysInfo OST to PST Converter. The software allows you to load the OST file, preview mailbox items such as emails, contacts, calendars, and attachments, and then directly migrate the data to an Office 365 account. Read more: https://www.sysinfotools.com/ost-to-pst-converter.php