Who first wrote the kernel for Mac OS X based operating systems?
It wasn't really Steve Jobs. Steve Jobs helped create OSX which is derived from the original operating system kernal.

OSX is based on the Mach Kernel. "Mach is an operating system kernel developed at Carnegie Mellon University to support operating system research, primarily distributed and parallel computation." Richard Rashid was the lead developer on the Mach project. Avie Tevanian was also one of the original developers on the Mach project.

OSX was then developed from this Mach Kernel by Steve Jobs' company NeXT. This happened during the time that Jobs was away from Apple. So Jobs' (company) developed the first iteration of OSX which was then purchased by Apple. Apple actually purchased NeXT outright, and Jobs went to serve as CEO at Apple.

Mostly from: http://en.wikipedia.org/wiki/OS_X, and http://en.wikipedia.org/wiki/Mach_kernel