If your same profile is not configured with Exchange Server, then you can not open the OST file directly. So I want to recommend, there is no successful manual method available to convert OST file to PST format. You should choose any third party solution, which easily convert your data with all data properties such as contact, calendar, to-do list, etc.
Here some websites, where you can pick this converter
Aryson, MailsDaddy, MailsSoftware, Datahelp etc.
I hope it will be helpful for you.