I recommend the third party KDETools software to convert OLM to PST file so follow this software OLM to PST Converter.

It consists of various advanced features by which users can safely convert OLM files to PST format. By using batch convert OLM files to PST format in a single click. Software permission to save data and migrate various file format like EMLX, MBOX, PDF, HTML, MSG, vCard for Contacts, ICS for Calendar, etc. the user can convert any file size as there is no limitation.

Read more information: https://www.kdetools.com/olm/