Setsuna,
Are you expecting this OS to be 100% standalone or to be dependant on the
current framework, which is dependant on Windows (with MAC & Linux ports
available)?
Yes you could create a basic operating system using C#, just remember C#
requires the Framework runtime. The Framework runtime currently requires
Windows. There are MAC & Linux ports of the framework available. (Rotor &
Mono).
Starting with something like the Rotor or Mono source, you could create a
100% standalone OS by changing any parts of the Framework & CLR to be 'self
dependant'.
Hope this helps
Jay