XNU (OS X's kernel) doesn't really use Mach as a microkernel. It supports user-mode device drivers, so you might call it a hybrid microkernel, but practically speaking, it's a monolith just like Linux; everything of practical relevance (file system, graphics, most disk and networking I/O) runs in kernel mode. Some parts (high-level USB, Bluetooth, power management, etc.) run as user-space processes, but they talk to kernel-mode device drivers.

(XNU was inherited from NeXT, which was based on Mach 2.5, which was actually not a microkernel at all.)