This article explains the different formats that use the CSO file extension. It also explains how to open each type, and how to convert a file type to a different format, so that it can be used in other software.

A file with the CSO file extension is most likely a compressed ISO image file. The format is also known as "CISO". It was the first compression method available for ISO files, and is often the preferred method for archiving PlayStation Portable games. The format supports up to nine levels of compression.

Although less likely, some CSO files may be compiled shader objects written in the Microsoft-developed High-Level Shader Language (HLSL).

You can open a compressed CSO image file with Format Factory, PSP ISO Compressor or UMDGen.