The CSO file extension is used to store compressed disk images in ISO format. They are mainly used by PSP - Sony's portable game console. CSO files can be created using one of nine compression levels. Depending on the height of the level, we get more compression, but also a longer loading time.

CSOs are files created with tools called PSP ISO Compressor, Prometeus, and YACC (Yet Another CSO Compressor). Games packed in CSO files are placed on UMD cards. They can be handled by the PSP provided that the user has the software to support this format.