There are various ways to create fog in OctaneRender®. The simpliest method is to add a Medium node to the Medium pin on the Daylight or Texture Environment node. In figure 1, A Volume Medium and a Scattering node are connected to a Medium Switch node to provide the ability to switch between each medium type connected to the Daylight Environment's Medium pin. The Post Processing Volume Effects can also produce fog effects with minimal effort. Please see the Post-Processing topic for more information. 

Figure 1: Medium nodes connnected to the Medium pin on a Daylight Environment node


The disadvantage of this simple method is that it does not provide much control over the density of the fog volume from foreground to background. VDB nodes provide more control over the fog density with scene depth.  In figure 2, a cube VDB has been imported using the Blender's Import OpenVDB volume file. 

Figure 2: Importing a VDB file 


After you import the VDB, create a new material with the VDB selected. This will create a material with the Volume Medium node assigned to the VDB. There is no standard setting for using a VDB file because the settings depends a lot on the volume geometry's size, the most important attributes to tweak are the Density and the Volume Step Length. 

Figure 3: Creating a new material the VDB