While underlying components of OS X are free/open source software, the top layers, such as the Aqua UI, are proprietary Darwin packages can be downloaded and compiled from the Apple Open Source website to make a bootable OS.
It shares none of the 'Classic' Mac OS design, and is completely rewritten and uses Next frameworks, a hybrid XNU/Mach kernel, and a BSD subsystem dubbed 'Darwin'.
As mentioned by Apple, Wikipedia, and others, it is said as Mac OS 10. It is the successor to Mac OS 9, hence the X signifying both its Unix roots and the major release version number 10. MacOS (formerly Mac OS X) is an operating system for Apple Macintosh computers, first released to the public on March 24, 2001, developed by Apple.
