Pretty easy to use - just make sure that Ruby is installed, then run this:
ruby opensong_to_onsong.rb ./path_to_opensong_file.xml > ./export_filename.txt
It just outputs straight to STDIO.
I just put this together pretty quickly, and by no means intended to cover the entire OpenSong spec. In my trials it seems to work pretty accurately. If there are issues - drop me a message and I'll look into them :)