[Dozy Van]

I am almost positive that all Operating Systems are Coded in C. If you want more info that than you need to ask a more specific question.

EDIT:

https://stackoverflow.com/questions/5802...written-in

Windows: C++, kernel is in C
Mac: Objective C, kernel is in C (IO PnP subsystem is Embedded C++)
Linux: Most things are in C, many userland apps are in Python, KDE is all C++