In the previous image, note the window to the bottom-left of the Viewport. This is the Node Graph Window, and looks identical to what you will find in the Octane Standalone. The nodes for all Octane Scene MaterialsA set of attributes or parameters that describe surface characteristics. will appear on this window as a node (or for more complex materials, a collection of connected nodes). The properties for a node appear in the right-hand panel.
This works exactly the same as the Node Graph Window in the Octane Standalone. For more information, see the topic in the Octane Standalone manual.