If you are finding an effective way to import MBOX to Office 365, I suggest using MBOX to Office 365 Migration Tool. It imports all MBOX items like emails, contacts, calendars, etc in the original hierarchy. It allows previewing the files before migrating.
Know how to migrate MBOX to Office 365:
https://www.msoutlookhelp.com/blog/import-mbox-to-office-365/