I don't believe I saw any Octree inside the source of PolyVox. I would recommend using one, as it can be very useful, here's a small list.
First, an Octree can be used as a 3D grid (yes, I know PolyVox has the volumes which are 3D grids already), but the advantage here is that it can compress voxels and load/store them in near real time to real time (just depends how you build up the structure of it)
Next, you can use Octrees to cut down the amount of Polygons you use per mesh. This is done by taking the root node and iterating through each of it's 8 branches and checking each of those ect. until you get to leaf nodes and then you can render the leaf node based on it's depth inside the Octree compared to the overall size of the Octree (e.g if the max size of the octree is 64 voxels in each dimension and you and the current leaf node has a depth of 2, you just take 64x64x64 / 2x2x2, which equals 32x32x32)
I'm sorry, I'm not the best person to explain things like this, but here's a link of what one can do:
Also note that this YouTube video is not mine, I just find it amazing
http://www.youtube.com/watch?v=km0DpZUgvbg