Software & Apps File Types CSO Files Your CSO file is most likely a compressed ISO image file Share Pin Email Print Evan-Amos / Wikimedia Commons File Types Design Cryptocurrency MS Office Windows Linux Google Drive Apps File Types Backup & Utilities View More By Tim Fisher General Manager, VP, Lifewire.com Tim Fisher has 30+ years' professional technology support experience. He writes troubleshooting content and is the General Manager of Lifewire. our editorial process Facebook Twitter LinkedIn Tim Fisher Updated October 24, 2019 If you run across a file with a CSO extension, you might not know right away what it is or how to open it. The answers depend on which type of CSO file it is. Types of CSO Files A file with the CSO file extension is most likely a compressed ISO Image file. The format is also referred to as "CISO." CSO was the first compression method available for ISO images and is often the preferred method for archiving PlayStation Portable games. The CSO format supports up to nine compression levels. The highest level provides the best compression but offers the slowest load times. Although it is less likely, some CSO files may instead be Compiled Shader Object files. These files are compiled files written in High-Level Shader Language (HLSL), which was developed by Microsoft. How to Open a CSO File You can open a compressed image CSO file with: Free PSP ISO CompressorFree Time's Format Factory UMDGenCommand Line tool Microsoft Visual Studio opens Compiled Shader Object files. Both PSP ISO Compressor and UMDGen are downloaded as RAR archive files. You can use a file expansion program such as 7-Zip (free) to open your downloaded file. How to Convert a CSO File Format Factory can convert CSO to ISO and vice versa. It also supports converting CSO to DAX and JSO, which are two other compressed image formats similar to ISO. UMDGen can convert CSO to ISO and DAX.