Yes, DataVare OLM to MBOX Converter allows users to preview OLM emails before exporting them into MBOX format. The preview feature displays mailbox folders, email, attachments & other details clearly. This helps users verify their data and select only the required folders or emails for conversion. It preview function allows you to make sure that you don't miss anything by having the ability to see what you will be converting. They will be able to be in control of all aspects of your conversion, as well as have confidence in the tool that you receive at the end. Fully works on Outlook editions and supports Windows versions. It also gives a free trial before buying.
Refer: https://www.datavare.com/software/olm-to-mbox-converter.html