There are lots of manual methods but they are very tricky and confusing so here I am suggesting you to use an dedicated solution to convert OST files to Outlook PST. In this regard, you can try EmailsGuru OST to PST Converter, which is 100% secure and easy to use utility for technical or non-technical users. The utility provide complete migration of OST emails, contacts, notes, tasks, etc. It is fully supportable to all edition of Microsoft Outlook. Anybody can try free demo version of the tool.