I’m sure we’ll be hearing tons of jokes about consistency from the spelling and grammar police, but I get why it would be MacOS, as opposed to macOS.
Mac is a proper noun. The Mac has always been capitalized. It’s a shortened form of a non-generic product: the Macintosh.
The “i” in Apple’s names over the years has always been lowercase, since the original iMac. So, of course it’s iOS.
For tvOS, Apple is using tv as a generic term. It doesn’t refer to Apple TV, just an operating system for your tv. Apple could expand this operating system to include more than just that puck they call Apple TV.
Same goes for watchOS. While the current product is called Apple Watch, a watch is a generic thing that has existed for a very long time. Who knows where Apple will take watches in the future; the lowercase “w” gives them the flexibility to do expand the category into many other products.
With the Mac, there is no generic equivalent. All Macs are called Macs, and it’s doubtful that Apple would ever put this os on anything that wasn’t called a Mac. Thus, MacOS.
Originally, of course, it was iPhoneOS, but since the iPad and iPhone share an OS, it only made sense to shorten it to the more generic iOS. ↩
Of course, it’s all OS X underneath. But that marketing term has long outplayed its usefulness. I can see Apple never making an OS XI, or whatever, at this point. At least not as a publicly marketed thing. Apple is not in the position it was in with OS 9, where its operating system couldn’t handle the modern features computers needed. OS X can simply continue to evolve to meet the needs of the newest technology, which is a testament to just how solid a foundation it was back in the late nineties. ↩