It was originally developed by Cox's company in the mid 1980s, and then adopted by Steve Jobs' company NeXT in the late 1980s as the official language of NextStep. The Apple connection is only that Apple bought NeXT and that its OS X is really just a Mac-skinned version of NextStep.
> OS X is really just a Mac-skinned version of NextStep.
You could probably describe Rhapsody that way, but by the time Mac OS X came out, I don't think that's an accurate characterization at all anymore.
In addition to the sizable Carbon subsystem, the NeXTStep pieces were also changed substantially, e.g. the refactor of Foundation that extracted CoreFoundation and the change of the graphics subsystem from Display Postscript to Quartz.
