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If you really want to convert the OST files to PST files, there is a manual solution that firstly exports the whole data of OST to PST format and then it can be imported to Outlook and can be accessed here. But this process would be full of data loss trouble and more complicated. So, to do a complete conversion of OST file to PST Outlook file here is provided VSPL OST to PST Converter Software that is best for recovery of OST included- emails, contacts, calendars, tasks, notes, inbox items, and other items and restore OST to PST Outlook file, EML, MSG, HTML, EMLX, MBOX, Office 365 and Live Exchange Server. For using the software, there needs to install an OST application and no require for an MS Outlook application to perform the conversion task. VSPL OST to PST Converter Software supports all MS Outlook versions included- 2019, 2016, 2013, 2010 & below. 

FREE Software- https://www.vartikasoftware.com/product/vs-ost-to-pst-converter-software.html