If you want to import OST file to Office 365, you should try
eSoftTools OST to Office 365 Converter software. Using it, you can directly import multiple OST files to Office 365 in a single process. This software can be used by all users without any facing data loss. Moreover, users can use it to quickly export Outlook mailbox items from OST to Office 365 and you can import up to 25 emails from each folder.
Read more- Import Outlook OST to Office 365