Yes it will run fine.
PCI-e is backward compatible. The bus will run at pci-e 2.0 speeds rather than 3.0.
It will just take a little bit longer to load your scene than if the mobo was pci-e 3.0 as well = no change for you but the actual rendering will be faster of course.
You can actually get 3 double slot cards in that mobo if they will fit in the case.
You need to look out for cooling issues with closely spaced cards though.
When you have more than 2 cards in there the bandwidth is shared between slots as per the notes.
They go from 2@16x + 2@8x to 4@8x, again really a non issue for Octane.
HTH
