There is no free way to convert the OST file into the PST format. However, if you only want to view the OST file emails, then I recommend, vMail OST viewer - is a free tool to view OST file data. Apart from that, you can also try the vMail Converter for OST this tool permits the users to convert first 30 emails from each folder of OST file into the PST format in free. If you want to save the entire OST file data, then you can purchase the full license of the software. 

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