I recommend using eSoftTools OST to PST Converter, which is the best tool to convert OST to PST files without losing any data. It can export data to various file formats such as Outlook, Office 365, EML, EMLX, MSG, HTML, vCard, Office 365, Gmail, etc. It can provide a free demo to all users in which they can convert an initial 25 items from each ost to pst files.
Get more info- convert OST to PST files
Get more info- convert OST to PST files