Disassemblies or source code of some of the non-MOS parts of the Acorn ROMs in the Master series.
https://github.com/stardot/AcornCmosBasic
https://github.com/stardot/AcornDmosBasic
https://github.com/hoglet67/BBCBasic4r32
http://8bs.com/basic/basic4.htm
https://github.com/hoglet67/ADFS/blob/master/src/adfs150.asm
https://github.com/dominicbeesley/ADFS-multi-target/
http://regregex.bbcmicro.net/dfs224.asm.txt
https://github.com/tom-seddon/acorn_1770_dfs_disassembly/ (based on the above)
https://github.com/SteveFosdick/edit4/
Nobody appears to have disassembled View.
Nobody appears to have disassembled ViewSheet.
Strictly speaking, these aren't part of the OS ROM, but close enough.
I haven't gone through this in any detail, but it looks like https://gtoal.com/acorn/files/arabic/Source/ might contain the original source for at least one of the companion ROMs.
https://gtoal.com/acorn/files/arabic/ https://gtoal.com/acorn/files/arabic2/