Sure there are more languages that are used to build operating systems, but C++ and C# are not one of those.
I was limiting the answer to the languages discussed here.
You probably could write an OS in C++ ... but why would you want to?
C# is managed code, intended only for end user applications.
Well, in fairness to the OP the original question was about what advantages OOP languages might have over C ...
My simple answer to that is "None".
They are different languages, with a different focus. C++ (for example) is bent on end user apps and re-useable code... C is more nuts and bolts, it's "closer to the metal" and far better suited to a message driven environment like WinAPI code. I've tried writing C++ apps using Windows API and it always ends up looking like C code...
Other OOP languages like Delphi and Oberon were major disasters resulting in massive program bloat to handle all the class stuff... like 1mb+ to play a midi file... something C can do with a couple of lines in under 10k.
So we're back to what Ralf said the other day ... Use the right tool for the job. If you're writing Windows API code, C is probably your best choice. If you're writing in a team with lots of ready to go classes, C++ makes the most sense... and so on.