ciso is a simple commandline utility to compress PSP iso files.
This package is an OSX port of a package provided by Ubuntu: http://packages.ubuntu.com/search?keywords=ciso
make
make install # (optional)
To decompress a cso file:
ciso 0 infile.cso outfile.iso
To compress an iso file:
ciso level infile.iso outfile.cso
where level ranges from 1 (fast, poor compression) to 9 (slow, high compression).