It depends what you call "OS"
A container/jail/chroot using anything starting from humble chroot, going through jail and openVz can use _DIFFERENT_ base libraries (/lib/libc and friends) and different binaries. So it all depends on your definition of OS. If Debian and RedHat are different OS then even chroot can run different OS.
It is possible to use it to run "same os" and people have used it like that. For example I wrote (jointly with then CTO of my company) in 1998 a set of extensions to Apache which allowed us to run each "user" in his own chroot and offer high end hosting. I believe PlusNet was offering something similar based on StrongHold in the UK.
However this is _NOT_ the way this was usually being used.
I used to run a system with 6+ chroots which was deployed to build & maintain different releases of a product. IIRC it had RedHat and several different epochs of Debian starting from 3.0.
Further to this, if you felt that psychotic you could actually run a passable Linux Userland in a BSD jail.