Until Octane is compatible with openCL (hopefully with Octane3) the 2015 MacPro is not compatible out of the box. You have to buy external Thunderbolt boxes like Akitio's to plug nvidia GPU's for the time being. If you're in a hurry, I would advise you to buy either a PC or an old "big and tall" macPro. You can fit PC nvidia GPU's in it quite easily.
Or just wait a few months for OpenCL compatibility.
MacPro 3,1 (early 2008) / 2 Xeon 4x2,8Ghz / 16GB RAM / 2 Titan X (1 Hybrid) / OSX 10.10.5 / Cinema 4D R17.048