How to Import OST files to Office365 Account-
vartika OST to Office 365 converter software
To import an outlook OST file into Office 365, convert it to Office 365 account first, then upload using the Microsoft 365 Import Service. To import outlook OST files to Office 365 account, first convert the outlook OST to Office 365 account, then use Outlook’s Import/Export or OST file into Office 365 migration tool.
https://www.vartikasoftware.com/product/vs-ost-to-office365-converter.html
I would suggest DRS OST to PST Converter Tool to import OST file to Office 365. This is the best migration tool. This awasome tool migrates all the OST file data to the Office 365 account without any hassle. In this tool, you are also given the option of date filter, so that you can migrate your email by filtering it according to a certain date.
You can use an OST to PST converter tool, such as Stellar Converter for OST Technician, to import OST files directly into the Office 365 mailboxes. This helps export all your offline Outlook data (.ost) files to Office 365 with no size limits and original folder structure and hierarchy. Import large OST files of 50 GB and more in a few easy steps into Microsoft 365; save time and efforts spent on the process. Specifically for Exchange or IT admins who are performing migrations or importing mailboxes from users whose accounts were deleted.
For more info- https://www.stellarinfo.com/convert-ost-to-pst.php