iphone os just like mac os x is derived from from Darwin is built around XNU, a hybrid kernel that combines the Mach 3 microkernel, various elements of BSD (including the process model, network stack, and virtual file system), and an object-oriented device driver API called I/O Kit. iphone os applications are built using objective-C just like mac os x. The aqua ui and cocoa abstraction layer is the same for both of them. The only difference being iphone uses cocoa touch layer (which is pretty obvious) I hope that clears up your misconceptions! Btw OS X , is just not another name they found via dictionary to name their fones and desktops/laptops operating system. Mac OS X, whose "X" represents the Roman numeral for "10" and is a prominent part of its brand identity, is a Unix-based graphical operating system. "X" in linux world stands for computer software system and network protocol that provides a graphical user interface (GUI) . Having a common ancestory( linux, bsd, nextstep) it was obvious to have such a name.