The octree properties are part of the scene object properties dialog, which is located at [Menu bar --> Tools --> Scene object properties]. You can also open the dialog with a double-click on an object icon in the scene hierarchy, or with a click on its toolbar button:
[Scene object properties toolbar button]
In the scene object properties dialog, click the Octree button to display the octree dialog (the Octree button only appears if the last selection is an octree). The dialog displays the settings and parameters of the last selected octree:
Voxel size: the size of the octree voxels. The smaller the size, the more memory will be used for the octree data structure.
Insert selected visible objects: allows to insert voxels based on the selected, visible objects. Only dummies, shapes, octrees and point clouds are taken into account. Select all objects you want to insert, then select last the target octree object.
Subtract selected visible objects: allows to remove voxels based on the selected, visible objects. Only dummies, shapes, octrees and point clouds are taken into account. Select all objects you want to insert, then select last the target octree object.
Clear octree: removes all the voxels from the octree.
Voxels have random colors: each voxel will have a random color.
Show octree structure: shows the octree data structure, mainly for debugging purposes.
Color is emissive: voxels will be shown in emissive colors.
Show points instead of voxels: for octrees containing a large amount of voxels, display can be quite slow. In that case you have the possibility to display pixel voxels, instead of cubic voxels.
Adjust color for next insertion: allows to select a new color, which will be used for new voxel insertions.