If you want to export OST files to PST format without data loss, you can use OST to PST converter software
. You can use this software even if the OST files are damaged. It is an easy-to-use application that requires no prior technical knowledge. Any technical or non-technical user can use this software with ease. You can quickly find master solution name of KDETools OST converter software to quickly works to convert bulky OST to PST Outlook format with Zimbra, Gmail, vCard, ICS, office MSG, EML, EMLX, MBOX, PDF, NSF, and etc. file formats.
more info- how to convert ost to pst online