I would suggest you use the SysInspire OST to PST Converter tool. This is the best software to open an OST file in Office 365. This software converts any OST file into PST and 15+ other formats including PST, Office 365, EML, EMLX, HTML, MSG, CSV, VCF, Yahoo Mail, IMAP Account, Gmail, Zimbra Desktop, and many more. The software also provides you with the best features like email filtration and mass conversion. And if you want to try this software then a free version is also available.