The Volume input of the Material output node doesn't do anything (neither does the Displacement input).
The Volume input of the material output doesn't have matching node/setup in Octane, you should be able to see this if you take a look at the generated NodeGraph (It will be there, but not connected to anything), and you shouldn't see the result of it in the viewport (right!?). (Also, technically: "Volumes" are called "Mediums" in Octane).
You have to connect your Medium node to a Material/Shader node (like Specular) to use it.
(I guess the devs just copied the Output node from Cycles and left "Volume" and "Displacement" there..)
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Octane Prime - Blender Plugin user