OS X isn't based on Linux, silly.
XNU is the computer operating system kernel that Apple Inc. acquired and developed for use in the Mac OS X operating system and released as free and open source software as part of the Darwin operating system. XNU is an acronym for X is Not Unix.
Originally developed by NeXT for the NeXTSTEP operating system, XNU was a hybrid kernel combining version 2.5 of the Mach kernel developed at Carnegie Mellon University with components from 4.3BSD and an Objective-C API for writing drivers called Driver Kit.
After Apple acquired NeXT, the Mach component was upgraded to 3.0, the BSD components were upgraded with code from the FreeBSD project and the Driver Kit was replaced with a C++ API for writing drivers called I/O Kit.
Like some other modern kernels, XNU is a hybrid, containing features of both monolithic kernels and microkernels, attempting to make the best use of both technologies, such as the message passing capability of microkernels enabling greater modularity and larger portions of the OS to benefit from protected memory, as well as retaining the speed of monolithic kernels for certain critical tasks.
Aren't I a clever penguin, see