Subdividing a planet's surface has approximately nothing to do with an octree. An octree is for volumes. Unless you are modeling the mantle and core of the planet, it is probably a bad fit.
The surface of the planet can be generated as a surface mesh or height map or similar.
In games many people start out with huge ideas of "I want many full real-size worlds!" Then realize that idea is extremely difficult. Even games like WoW don't reach the content size of Earth. A few seconds on Google says:
* Currently the WoW with all expansion packs is somewhere between 45-60 square miles.
* Skyrim's actual navigable overworld is actually on the order of 15-20 square miles.
* Dragon Age Inquisition has two large kingdoms on the map and feels bigger than Skyrim, but is about the same 15-20 square miles of navigable space.
* GTA 5 is about 100 square miles but has a ton of dead space and repeated patterns throughout, it is not dense.
The games do a lot of lying to convince you the world is really a big world. The biggest games have models that fit inside a real-world city. Not that it is small, that is an enormous amount of content, but it should give you a sense of scale.
Those are games where the budget is easier to measure in billions of dollars rather than millions. $0.25B, $0.31B, $0.26B, etc., and they produced under 100 square miles. Earth has about 60 million square miles of land.
The biggest problems with world-sized games is trying to generate compelling content.
