Can probably use Audacity. May need to install a ffmpeg plugin first to read the opus files, then:
goto Tools > Apply Macro > Palette…
select “MP3 Conversion”
then click “Files…” to choose your files and it should start a batch process.
Before that, probably go to Edit > Preferences > Directories and change the entry for “Macro output” to a custom folder so it’s easy to find. (note: the instructions on that settings page says to leave a field blank to use the last directory, but that’s not the case for macro output, definitely set a custom location for that.)
edit: also a good method for converting FLAC if there’s a need for it