OLM files can’t be changed to MBOX until a paid software is used. I have worked with quite many Converter tools in the past. Each tool offered a different set of features, even though they all offered the same conversion. Among all the brands, I liked the working of Gladwev’s Products. The applications are too fast at converting emails with the desired accuracy. They have a tool named OLM Converter Pro. This tool can Import OLM to MBOX files.
You can try another option of a third-party tool for the Mac OLM to PST Conversion as that can make your conversion easier faster and safer. You can try this OLM to PST Converter for Mac in any Mac OS Version. Visit:- www.toolscrunch.com/mac-olm-to-pst-converter.html