OJOS (the Original J Operating System), aka Platypus is an OS which .. aims to be a small experimental, extensible kernel that provides a hardware abstraction layer and loadable module support. It is designed to allow new ideas in operating system theory to be tested easily. The Platypus version will include the microkernel, pieces of the hardware abstraction layer, multitasking, and memory management. [..]

Quite interesting is the idea of a Forth/Lisp mix for system programming (see the J-Language on the site below; see also RPL and POP-11).