Anyone that actually programmed MS-DOS, knows that we used to program directly against the hardware for actual work. MS-DOS was nothing more than what is usually known as monitor in OS literature.
Continuing the texts from people more relavant to the CS world than me,
"An operating system is a collection of things that don't fit into a language. There shouldn't be one." - Dan Ingalls on
"Building Parallel, Embedded, and Real-Time Applications with Ada" - John McCormick
"Project Oberon: The Design Of An Operating System And Compiler" - Niklaus Wirth
And not to let this just be theory, here are a few examples of commercial products using the language runtime to interface with the hardware.
Like I said, I'm not interested in debating the definition of OS. Best regards.