The Grove is a small part of Oasis that is surrounded on all sides by trees and cliffs. It is here that you can talk to Harold. This area can only be accessed after the Lone Wanderer has taken part in the ritual. It is entered via a wooden gate that comes from the pavilion, after it has been unlocked. For entering the Grove the first time, the Lone Wanderer must have followed the ritual of purification, where they drink the sap from the tree and go unconscious. When waking back up, the Lone Wanderer will already be in the Grove.
In the middle, the talking tree, uniting Harold and Herbert/Bob, can be found. To the south-east, a small radioactive pond can be found. If the Lone Wanderer dives in the pond, he can see an Easy locked hidden underwater door which leads to the damp cave.
The Grove appears only in Fallout 3.