Fallout 3. Studies of a post apocalyptic suburbia by Adam Adamowicz.
A settlement located north-west of Clifftop Shacks in the Capital Wasteland. It is unusually full of trees and green plants. Birds can also be heard once inside the gate, suggesting that animal life has since been flourishing here.
Oasis was originally a mine, but after the Great War it became just another part of the wastes. This was until Harold, a special mutational product of the Forced Evolutionary Virus or FEV, arrived.
Harold possessed a tree growing on his head which he called Bob and eventually found himself overwhelmed by Bob and became rooted to the ground during his travels in the Capital Wasteland region after 2241.
Here, he was discovered by several people who began to worship him as a god, and a small and exclusive cult known as the Treeminders began to form in secrecy.
I totally flipped my shit when I realised this was THE Harold.