RiscOS 4 has been freely available and partially open source (and under fairly active development) for years now. RiscOS 5 was the subject of a weird licensing situation for a long time and therefore hasn't seen nearly as much active development. RiscOS 4 runs happily on all models of Raspberry Pi along with similar boards as a native OS, although it suffers from all of the problems that independent OSes run into, such as poor device support and applications which don't support modern standards i.e. javascript.
That being said, you can still drop into BBC Basic from the desktop.
Korg AG-10 | Kawai XS-1 | Roland CM-32P | Yamaha FB-01 | Roland D-110 | Roland M-GS64