I was hoping to be able to have this post instruct you to use PINN instead of NOOBS, b/c I think there have been cases where PINN's list of available OSes was more extensive than that provided by NOOBS; this being one of the many advantages of PINN over NOOBS.
Unfortunately, I just loaded up the latest PINN (from 7/13/17) and discovered that, indeed, RISCOS is missing from the list. I wonder why... I know it was there in the past. So, I'm not sure what I can tell you.
FWIW, I do have some vague recollection of there being some kerfluffle here to the effect that there was something wrong with RISCOS (maybe that it didn't work on the latest Pi hardware - i.e., the 3) and that, for some reason, it was being "taken off the market". But I don't remember any of the details of that discussion.
Just out of curiosity, why do you want RISCOS? I have to admit that I've installed it a few times, played with it a bit each time, said things like "Yeah, that's cool", but could never figure anything I actually could *do* with it. It seemed like most of the effort as getting used to its windowing model, which is pretty different from what we are all used to. Again, to be clear, I really *wanted* to like it; I just couldn't make myself do that.
If this post appears in the wrong forums category, my apologies.