If you want to recover that Outlook OST. Which cannot be opened in Microsoft Outlook error, then go to this eSoftTools OST to Office365 converter software and import the Outlook OST file to Office365. It is an extremely simple and comfortable tool to use which works very easily. It converts OST files to PST files and many different file types like EML, HTML, Office 365, ICS, MBOX, Zimbra, EMLX, MSG, Yahoo, Gmail, and vCard. This software has a live preview option that helps you see the preview of OST in Office365 before conversion.