For some reason I am unable to select the check box for my GTX 780 in the Blender Octane Render devices preferences box. A red X flashes in the box when attempting to select it. This happens even if I uncheck my other two graphics cards - Blender Octane simply will not allow me to select my 780 (see attached movie screen recording).
I do not have this issue in Octane Stand Alone 2018.1 RC6. I have two GTX 1080 ti's and one GTX 780 and can use all three cards there.
FYI - don't know if it matters but I am not using the most recent NVIDIA drivers. I currently have installed 416.34