You can easily Open an OST file in Outlook 365 by using SysInspire OST to PST converter tool. This is one of the best tools for conversion. It can convert OST to PST, EML, EMLX, Office 365, etc. it has a feature of split option you can split PST file in multiple small sizes, it has an email filtration option that can help you to restore emails selected by date range. It has technical support 24x7 with chat option. It can convert corrupted OST files into PST. It has a demo version that can help you to convert 25 files from each folder.
