Hi, I would suggest using the Unique & tool available in the market which allows users to transfer OST files to PST, Zimbra, EML, MSG, MBOX, OLM, Gmail, Office 365, etc. as per the user requirements. Also, it consists of several advanced features by which users and wonderfully restores the converted OST file into PST, Zimbra, MBOX, vCard, vCal, EMLX, EML, MSG and HTML file formats.
More info- 100% free OST to PST Converter
A few days ago i have also suffered this issue then my friend suggested me try this OST to PST converter software and i tried that software and got a great result and that tool is easy to use and very secure. you can download the demo version of this software which converts few items from each folder.The application is workable with all Microsoft Outlook versions. I hope you will get the best results using this software. you can download this software from here:-https://www.betavare.com/ost/pst/