The easiest way to open a CSO file is to double-click the file and let the PC decide which default application should open it. If it does not work out, your PC may lack an application which can open or edit CSO files.
You can try the following programs to open CSO files on your computer.
You can also try to use Notepad or another text editor to open the CSO file. Many files are text-only files meaning no matter the file extension, a text editor may be able to properly display the file's contents.
Warning: Take great care when opening executable file formats received via email or downloaded from websites you're not familiar with.