The premise is a stretch. Arguably, general purpose computing today is either nix or VMS-derived--and I'm not even sure those are are sufficiently different in underlying model to be completely separate operating system trees.
That said, I can buy Windows NT as sufficiently distinct to be a unique 1990s OS. But Linux is clearly a *nix and OS X is as well--UI and integration notwithstanding. Sure, you can pick NeXT Step and Linux and go "90s!" but they're clearly part of a much earlier tree.
And, if you bring in mobile, Android clearly derives from Linux. I don't know enough about iOS internals to identify where it sits in the OS tree.
And NeXTstep isn't a 1990s OS. It's a 1980s OS. v0.8 first demoed 12 October 1988 when the NeXT cube was launched; v1.0 shipped 18 September 1989.
NeXTstep influenced the Windows 95 UI in some ways -- the shaded 3D look, the idea of a fixed panel across the screen which could be both an app launcher and an app switcher.
But NeXT probably got that from Acorn RISC OS, which was shipping before NeXTstep was ever shown.
That said, I can buy Windows NT as sufficiently distinct to be a unique 1990s OS. But Linux is clearly a *nix and OS X is as well--UI and integration notwithstanding. Sure, you can pick NeXT Step and Linux and go "90s!" but they're clearly part of a much earlier tree.
And, if you bring in mobile, Android clearly derives from Linux. I don't know enough about iOS internals to identify where it sits in the OS tree.