In Odyssey, you will be able to fly down to planets, fly through atmospheres, and drive along planet surfaces in your SRV; all while remaining in VR.
When disembarking your ship or SRV, you will be presented with a projected flat game screen in your VR headset in order to continue on foot. You will of course be able to remove your headset if you so choose, but this will not be a requirement.
There will be no requirement to boot between Horizons and Odyssey in order to continue your journey.