The best way to convert any OST file is by using the SysInspire OST to PST Converter tool if you are having difficulty to open OST file that cannot be accessed. To convert your OST file to the various formats you want to be, including Gmail, MSG, Office 365, PST, IMAP Account, CSV, VCF, Yahoo Mail, EML, EMLX, HTML, Zimbra Desktop, and many more, you can use this software. Due to its simple interface anyone can use, you can also use this software without any technical experience. Its free version is also available, which lets you convert 25 things from each OST file and folder.