This is the simplest method for exporting OST to MBOX. This software for converting OST files to MBOX works with every version of Microsoft Outlook, including 2000, 2003, 2007, 2010, 2013, 2016, and 2019. You can directly export all of the data in your OST file into your PST using this software. With complete precision, you can convert your OST file to PST, EML, MSG, MBOX, ICS, VCF and other file formats. You can free download the demo version of this before purchasing the full license version. 

This software is available for download here: https://www.perfectdatasolutions.com/en/ost/ost-to-mbox-converter.html