Want to write here for anyone who reads in the future: If you want to go 10980XE in an ITX form factor with air cooling, use these:
The remainder of the parts are fairly simple: NVMe SSD + 850W PSU + any AMD GPU supported by macOS. Aaand as an Easter egg... check out this crazy ITX mobo! LGA 3647 for a Xeon CPU. Wild.
Air cooled stuff then but keep in mind it would be good to blow it out every once in a while with an air can to clear that dust out, as that's a good thing to do even if you own a console (or other electronics, I blow my monitor vents every so often too for example) keeps things cool and whatnot. Noctua this is by a quick search really good but expensive otherwise the Evo 212 is good too and way cheaper.
I'll just preface this by saying I spent a lot of time browsing the Octane Render forums when I was trying to figure out my build. I don't have links to any threads but there are plenty there discussing system requirements for the renderer.
These are just some of the glaring mistakes. I would recommend you spend way more time researching parts as it is clear that you have spent very little time doing so. It took me around a week of extensive researching to put my build together. Expect to spend the same time doing so.
Get a Noctua cooler. Look at the Amazon review breakdown of this one. 92% 5-star reviews our of 409 reviews. I'm not sure if this one will fit for you though.
https://www.amazon.com/gp/aw/d/B00L7UZMAK/
You're wasting money on that memory. Your motherboard only supports up to 3200 MHz memory. Your CPU only supports up to 2133 MHz.
https://www.asus.com/us/Motherboards/X99DELUXE/specifications/
http://ark.intel.com/products/82931/Intel-Core-i7-5930K-Processor-15M-Cache-up-to-3_70-GHz
Get this memory instead.
ASUS has a new version of your motherboard anyway. I wouldn't buy the old one.
https://www.asus.com/us/Motherboards/X99-DELUXE-II/specifications/
Do you need the Deluxe? Look at the non-Deluxe version below.
https://www.asus.com/us/Motherboards/X99-A-II/specifications/
If you're using Octane you could use up to 4 video cards with a different motherboard.
https://www.asus.com/Motherboards/X99E_WS/
Although, if you went that route you would have to do liquid cooling and it would probably only fit in the biggest CaseLabs case. A system like that would run you over $10,000. Such a system would only be necessary if you were working and rendering by yourself on a single system. If that is not the case, 2 cards will suit you just fine.
Western Drives are more reliable. Read the Amazon reviews of this one.
https://www.amazon.com/dp/B011LVAVEQ/
The choice of power supply is great.
If I was going to do an air cooled system, I would go with this case. I'm not sure if you need it though.
https://www.amazon.com/Cooler-Master-HAF-Computer-RC-942-KKN1/dp/B003S68Q0Y
EDIT:
With all that money you're saving on memory you could get a better CPU.
http://pcpartpicker.com/product/NXyxFT/intel-cpu-bx80671i76900k
This CPU supports up to 2400 MHz memory, meaning you could also upgrade your memory.
http://pcpartpicker.com/product/YK8H99/gskill-memory-f42400c15q264grk
I'm not sure about the case or how you're cooling the GPUs. I know from researching on the Octane forums that anything above 2 cards needs to be water cooled otherwise your cards will overheat and you will lose performance. Look into the founders edition because of its blower-style cooler.
EDIT:
I forgot to mention that when I was looking into this for myself I was trying to build a system for animation. If you're only using Octane for stills 2 cards will be great.
I'm not sure that's correct. The Amazon page lists NM-i17xx-MP83 as coming with the new NH-D15s.
Going directly to the source, check out this guide from Noctua, which shows MP83 is what you want for the NH-D15.