I want to suggest to you the best OST file conversion tool that name is PassFixer OST to PST Converter software. This tool is very advanced, that tool helps you to convert big size Outlook OST file to PST and many other formats such as EML. HTML, Yahoo, Gmail, Office 365, MSG, ICS, MBOX, Zimra, vCard, and EMLX. you can also get multiple features to convert Outlook OST files fastly. The live preview feature helps you to see the preview of your Outlook OST file at the same time. Download the free demo version of this tool and get 25 mailbox items from each folder.