I understand that you want to migrate the OST file to Office 365. You can use MailsDaddy OST to Office 365 Migration tool. This software quickly converts all the OST emails with meta properties (To, Cc, Bcc, Images attachments, contact, calendars, etc). It imports OST files to Office 365 mailboxes with impersonation. This software supports all the versions of Windows. In the free version of this software, you can convert 20 items for free. You can also check the performance of this software by using its trial version on your own.
Read more - https://www.mailsdaddy.com/ost-to-office-365-migration-tool/