If you are searching for a 100% safe & secure solution then I recommend the A2Z Migrations for OST to PST. Using this OST to PST converter you can simply export whole OST emails, contacts, calendars, appointments, tasks, and attachments, etc into the Outlook PST. It allows multiple export options like MSG, EML, HTML, and RTF, etc apart from the PST file extension.

To get more information regarding the tool, visit: http://www.a2zmigrations.com/ost-to-pst-converter/