I'd be surprised if Windows 10 didn't end up with more installs than all versions of all non-Windows OSs running on computers within 3 months of its release. It sounds like a pretty solid upgrade over both 7 and 8, plus it's a free upgrade from 7 or 8.
Within 3 years of the release of Windows 10, I'd expect it to be installed on 70% of all computers (and for some small percentage to be running 7 and 8.) OS X may be growing every year, but going from Windows 7 or 8 to 10 is going to be a pretty easy transition (download and install, for free) compared to moving to OS X (buy an expensive computer, then figure out how to transfer all your old stuff). So I'd expect Windows 10 will grow far faster than OS X.
Having said that, I'm surprised Oculus Rift thinks that OS X is such a small market as to not be worth targeting. I can't find any numbers on the matter, but I'd expect a large number of people in game development (less than half, but more than you see for general computer usage) using OS X. So they're shooting themselves in the foot.
OS X is also a free upgrade, and will be free in perpetuity, while Windows 10 is only a free upgrade for the first year it is on the market. And judging by the Technical Preview, which is available, it is chock full of bugs, and it may take more than a year for these to be worked out. So by then, it won't be a free upgrade anymore.
The latest version of OS X, currently 10.10.3 Yosemite, will run on hardware a few years older than the current models, which allows for a sizable percentage of Mac users to install it. So, I don't think this will be much of an issue, but I do agree with you that there is a big installed base of OS X gamers, more than the tech media is talking about.