The safest method to upload OST files to Office 365 is using the OST to Office 365 Migration Tool. It is the most recommtended and trusted tool. It directly converts OST files into Office 365. It has many advanced features which let you import your files including emails, attachments, and contacts. It supports OST files of Outlook 365, 2019, 2016, 2013, 2010, and older versions. The software is compatible with Mac and Windows OS.