Windows, OS X, RHEL all those are operating systems; Linux is not. It might seem pedantic, but one must suggest distro's: CentOS(Use RHEL docs), Ubuntu(I recommend Ubuntu Studio for Desktop), Debian, Fedora, GNU/Linux/.*. I might even be as progressive to name freebsd, pc-bsd, RaspBSD.
But sorry, I'm just unwinding a bit on HN, after work.