I guess technically that'd be Arthur rather than RISC OS.

My high school had a particularly well connected compsci teacher who managed to get his hands on one of the first Archimedes to be sold (using his own money it must be said ...). At that stage it shipped with Arthur because, or so we were told, the "actual" OS was still in development. As it turned out, that OS was eventually cancelled and instead Arthur was upgraded (in practice ... replaced by) RISC OS.

I still have fond memories of playing the demo that would become Zarch (aka "Virus" on the Amiga/ST/Spectrum/etc.).