The ultimate solution to convert OST to PST format is to use Recoveryfix for OST to PST Converter which can convert single or multiple Outlook OST files, including Emails, Attachments, Contacts, Distribution lists, Calendars, Tasks, Notes, etc., to PST format. The software migrates OST files to Office 365, Live Exchange, Outlook.com, Gmail, and other IMAP servers.
link-> https://www.recoveryfix.com/ost-to-pst-converter.html