Who invented Mac OS?

Mac OS was not exactly an invention. Mac OS was created by simplifying and reducing in size the OS that had been created for the unsuccessful Apple Lisa computer. Mac OS X was created by adapting the OpenStep OS created for NeXT Computer's NeXT computer. OpenStep OS is based on BSD Unix (going back to the 1970s at UC Berkley) and the Mach Kernel (going back to the 1980s at Carnegie Mellon). The development of Mac OS is more of an evolution than an invention.