Fionn, Unix was developed at AT&T Bell. I'm not sure if it ever was truely open source although at one time I believe it was at least freely available to academic institutions. There is an open source varient called FreeBSD which I think inherited from Unix code. Linux was a write from scratch job but is pretty compatible.

As for your point about running OSX programs on Linux, I suspect you would need an emulator as you do with older Mac programs. Basilisk II could be worth a try in both cases as I read there is now an OSX port.

Jon