In Coilbook's image you can clearly see fog at the bottom of his red circle, that is not alone a Post Process effect, although, it may have been enhanced by it.
But, functionally, you do that just by placing any old VDB shape at the base of mountains with low density to get that, right?
I think Milanm is right, you would have to compare a simple same scene render of Arnold vs Octane to identify a difference between the 2.
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