Do I?:
1 take the Oculus and hope they downplay the recommended specs
2 take the Oculus and pray there’s a dedicated gaming machine (XB1/playthingy/wii) coming soon - tm
3 get a refund and buy a classic car and wait till VR is sorted
4 sell the iMac and go back to Win and tweaking drivers for the rest of my life!
4a there’s another option; my other half has to get a laptop but hey.. water cooling GPU’s (plus £1000’s) seem the only option for a GFX card that doesn’t have an ’M’ after it - are there laptop/notepad options on a budget?
As much as you may not like a windows machine, and no I am not preaching it is better, it is the same hardware that is in both, but the answer generally is 'use windows for gaming' though you might be able to look into an alternative, your CPU is plenty powerful, and you can get external graphic cards docks now a days, buy one for your thunderbolt imac, and get bootcamp or similar so you can boot both mac and windows at the same time, and use windows exclusively for gaming, might be a cheaper alternative, and will allow you to keep your mac as is with near no reason to upgrade for quite some time.
Advice given, a lot of the things people feel about windows, at least in my experience don't happen, it seems to be a fear/prejudice that people seem to build up over time, maybe because they have only used OSX and iOS? I don't know, Windows and OSX are functionally more or less the same the main difference really being compatibility.
With windows, you can more or less buy any product and provided you find the right drivers (assuming they don't come with product) it will work.
With mac, you need to buy a product that supports mac, but then its just plug and work, note that any windows stamped device does the same with windows has for a while now.
Windows 10 will have dx12, vulkan and most everything else you can create of graphic api's.
Mac/OSX will only support Apple's own "metal" API, that's it no vulkan no nothing.
Does that make windows 'better' then OSX? well they use the same hardware now, so it is only the software that is the difference, and to me at least the limitations OSX would give me in my usage, are not appealing, others might not hit those limitations or mind them or whatnot.
Use what is best for you

that you are most comfortable with, but try not to write something off because of prejudice and a few bad experiences, bad experiences can happen everywhere, sure it might not happen on OSX for you, but it does for others, and same with windows, so use what you enjoy

just know that windows isn't as bad as some make it out to be, from my experience repairing computers, Window's simply allows for many more user errors, but they are mostly, user errors. OSX limits people, so there's a limited amount of user errors there, a good thing when things need to just work though definitely, but the reason it usually doesn't "just work" is user error.