Compressed .ISO disk image format used for archiving PlayStation Portable (PSP) games; may include up to nine levels of compression, but higher amounts of compression may cause the disk image to load slowly.
Program(s) that open cso files
Mac: Prometeus Windows: UMDGen; CiSO Converter; PSP ISO Compressor
Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/cso
