Tripos, actually. Most of it was implemented in a programming language know as BCPL (precedor of B which is a precedor of C), which AmigaOS 4 finally have got rid of. There were never many BCPL-fans (Amiga Guru Book says that there were two)... TRIPOS is actually still developed.
Tripos was actually a backup plan. Here is a very good article by Andy Finkle about CAOS: The original project, not based on Tripos:
http://www.thule.no/haynie/caos.html