Try the mentioned Outlook OST file Converter tool from KDETools software. This ost file to pst converter online can export OST to PST file without installing MS Outlook without create any issues. Through its tool you can transfer OST to PST, Office 365 account, pst file, EML, EMLX, MSG, HTML, MHTML, MBOX, VCF, CSV, ICS, Live, Exchange Server, Zimbra, Gmail, Yahoo, and etc
OST converter

How to  Transfer OST to PST
How to Convert OST to PST Outlook Manually

Outlook OST file Converter tool 

http://www.microsoft-ost-to-pst.com/