Jobs founded NeXT, which created a desktop UNIX OS based on mach/BSD. Most importantly, NeXTSTEP had a nice GUI framework built on Objective-C. When Jobs returned, NeXT merged with Apple, and Apple gained ownership of NeXTSTEP/OpenStep, evolving it into Rhapsody and OS X.

Of course the programming and design were done by the software teams under Avie Tevanian, first at NeXT and later at Apple.