Hiya,
The only kind of 'open source' you get with the classic side is when people reverse engineer something (which obviously takes ages).
I guess in an ideal world, the owners (whoever the hell they are) of the original OS could have released the source code to the community after they'd finished developing it (around the year 2000).
The other problem is that a lot of the original OS was written in 68k assembler which doesn't lend itself to being developed further (due to a lack of skilled asm coders).
It sounds like Hyperion have spent the past decade converting all of that cryptic assembler code to straight C code. I guess that makes them the only entity that's really in a position to open source Amiga OS.
Then of course you have AROS which has taken a different approach (built from the ground up).