This is a bit complicated because NeXTstep didn't use a BSD kernel, using the Mach microkernel instead. Much of the layers above were taken from BSD, so it definitely had a BSD feel to it, but it wasn't a BSD kernel so maybe couldn't really be called a BSD derived OS?
Mach did not separate BSD into userland (like a true "microkernel") until 3.0. NeXTSTEP did not use Mach 3.0. Mac OS X still has the BSD code in the kernel.
