Operating Systems in CC are the pieces and names that hold the computer together. The OS is determined by the BIOS file, the images, the help pages and the ROM. The BIOS determines core things, like the OS files, APIs, names,terms and built in mechanics. The images simply make the look of the computer. The help pages and ROM determine how it functions on the user side and make it seem different (exception of the images).
The OS can be changed, but that is not an easy thing if you plan to make any significant changes. OS enhancements can be made, but are quite hard to install. There is one simple way, and that is to get a zip file in the "Computer Craft" folder and unzip it and let it redo everything.
It is technically possible to add in new programming languages using Lua to make a compiler, but that hasn't been done yet. An OS is a combination of features, to retell and redo what the computer is, like Windows, Mac and the Linux distributions.