The Shadow Catcher option can be used to create shadows cast by objects onto the surrounding geometry. The shadows cast are not limited to simply a ground plane but can be cast onto other surfaces of varying shapes.
Shadow catcher is enabled by activating the Matte option on the Diffuse material. This means that the object(s) receiving the shadow should have an Octane Diffuse material applied to it.
In the Render settings window, activate Alpha Channel and disable Keep Environment. When the image is rendered, the shadows will appear over the transparent parts of the surface. This image can then be used in a compositing package to merge the object and the shadows into the composition (figure 1).
Figure 1 : Shadows appear in the transparent parts of the image where the shadow catcher objects were placed