The Grayscale Vertex Attribute node operates similar to the Color Vertex Attribute node. The difference is that this Node produces a float value from the vertex paint data to use for MaterialThe representation of the surface or volume properties of an object. attributes such as Opacity. Set the corresponding Name parameter to MapChannel:-2 as the default channel for Opacity.
Your mesh object will need to apply the Octane Geometry Properties modifier to successfully read the MapChannel properties correctly.
Figure 1: Setting the Grayscale Vertex Attribute node's Name parameter to -2