How closely are Mac OS X and BSD related?

Question

I read that Mac OS X and bsd are related. How closely are they related. Can Mac OS X software be tweaked and installed on BSD?

Answer 1

Installed - no.

Ported from source, maybe - see Gnustep, Cocotron, EtoileOS - all of which offer varying degrees of compatibility with the Cocoa development stack (but not with the older Carbon). There is a limited amount of x-platform objective-C software developed that way.