You can search easy third-party tool of KDETools for OST to PST converter because I already use its third-party tool I think few days ago. The software to convert database from offline OST file into PST files and also keeps original Emails properties such as- To, Bcc, Cc, Subject, and etc.
Conversion the OST mailboxes to PST, follow the simple procedure:
• Go to Files >Open & Export and click Import & Export.
• Add Export to a file; then click Next.
• click the second option—Outlook Data File.
• Select the folder which you want to convert and click Next.