Apple’s lineup of operating systems all have coinciding names to their appropriate hardware names: iOS is on iPhones, iPads, and iPod touches, tvOS is on the Apple TV, and watchOS is on Apple Watch. However, OS X doesn’t really make itself clear unless you refer to it as “Mac OS X”. A complete novice to Apple technologies in my opinion would never guess that OS X was assigned to the Mac. However, it looks like this might change with a total rebranding of OS X to “macOS”.
According to a Brazilian blog known as MacMagazine has discovered an Interface Builder document buried deep within OS X’s System folder which refers to OS X as “macOS”. Developer Guilherme Rambo was able to uncover the document with the reference within the FlightUtilities.framework by going to /System/Library/PrivateFrameworks/. We were able to confirm that this is the location where you’ll find the macOS reference, hence our screenshots below.
The file is specifically called FUFlightViewController_macOS.nib and, at least according to Rambo, this name change was made as a part of OS X 10.11.4 which is quite interesting as this version of OS X is likely one of the last for El Capitan before OS X (macOS?) 10.12’s beta release this June at WWDC 16.
It’s worth noting that developers tend to refer to OS X as macOS pretty often in their apps for convenience purposes since they’ll be able to immediately identify a Mac app if it’s tagged “macOS” and an iOS app if it’s tagged “iOS”. Apple may be using the same technique here, however it’s odd that they now just start using it this close to WWDC which would likely be the place we’d hear the company say they’re rebranding OS X.