|This page is about the location. For the quest with the same name, see Oasis (quest).|
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The innocent creatures are drawn here by His gifts. They come from across the Wasteland to live in this sanctuary. This part of the world is healing, my friend, and it's all thanks to Him.”— Tree Father Birch
What is known as the Oasis was once just one of many mines scattered across the landscape of the United States. The extensive network of caves beneath it was used as impromptu shelter by the inhabitants of a nearby mining town from the bombs. As the time passed, the sheltered rocky vale was stripped of mining equipment and other installations, with only a handful of artifacts surviving to the 23rd century.
Sheltered by the rock formations in the northern Capital Wasteland, it gained a new lease on life in the 2240s or 50s when Harold found himself overwhelmed by Bob in his travels and laid down roots in this area in what the Treeminders call the Harvest Month (when Bob's seed pods ripen and disperse their seeds, resulting in the creation of new trees, grass, and plants).
With Harold's mutation progressing further he became the unwilling heart of an oasis in the desolate wastelands. Around 2257, a small cult formed under Birch, dedicated to the worship of Harold. Birch saw in Harold not an unfortunate mutant condemned to live out his life as a tree, never again to resume his wasteland travels, but a god. The cult, the Treeminders, were to be his last line of defense.
The Treeminders shun technology in favor of nature, blaming technological progress for all human misery. The control over the cult, exercised by Tree Father and Leaf Mother (Birch and Laurel), with equal say in its affairs. As the Oasis grew and nature once again reappeared in the wasteland, the cult confronted the hostility of the wasteland. Although they disdained technology, the Treeminders continued to use firearms, to remain on equal footing with their enemies. Only select outsiders were permitted to live long enough to enter the Oasis, though it was rare. Most are turned away, some are killed when they try to gain entry by force, and others are shot on sight, like raiders.
The Lone Wanderer in 2277 is the first one in years to visit the vale. The Oasis is unlike any part of the wasteland, filled with lush vegetation, trees, and even birds living within the sheltered forest. No other place in the wasteland is so teeming with life.
The Oasis is located inside a large rock formation in the north of the Wasteland. The gate to enter it is accessed via a narrow passage in its western face.
The Oasis itself is a small vale located inside the rock formation. It's a small area, centered around a makeshift pavilion with the basin of purification. Scattered inside alcoves created by the rocks and trees are resting places for the Treeminders.
 Harold's Grove
The grove where Harold rests is a small area centered around him. 
 Oasis caves
 Sunken chambers
The lowest level of the caves below the Oasis is partially flooded. It also contains a survivor camp with the remains of a holocaust survivor and, of course, Harold's heart.
 Notable loot
- D.C. Journal of Internal Medicine: Inside a wooden box in the survivor's camp.
- Pre-War book: On the blue trunk in the camp.
- Stealth Boy: On the table in the camp.
- Mini nuke: Standing on a rock, underwater, inside the northern chamber
 Damp cave
The damp cave is a single, large chamber with several mirelurks, some minor loot, and a direct exit to Harold's grove.
 Notable loot
- Nuka-Cola Quantum: In the northern inlet in the cave.
- sniper rifle: In the eastern inlet, next to an ammo box
- Three Dog mentions the Oasis on Galaxy News Radio, as a tree-filled place, though he mentions it was possibly in a drug-fueled haze.
- The Preacher from Seward Square will yell about trees in the north, referring to the Oasis.
- The water in the Oasis caves is irradiated from the Great War.
Oasis appears only in Fallout 3.
- Linden's dialogue: "Any information you could give me on the caves below Oasis would be helpful." (The Lone Wanderer)
"Oasis is located near what used to be an old mining town before the bombs fell. When the attacks started about 200 years ago, many people took shelter in the natural caves that dotted the area. The old gate that stands at the mouth of Oasis Cave is the last piece of architecture from those days."
- Harold's dialogue
- Poplar's dialogue: "How is it you're causing all of these things to grow around you?" (The Lone Wanderer)
"What's causing this place to grow?" (The Lone Wanderer)
"In what we call the Harvest Month, the Great One creates seeds among his branches within seed pods. At the end of this time, the pods open and the seeds are carried quite easily upon the wind. It's quite beautiful. Wherever the seeds take purchase, they grow into trees, plants, grass or all manner of wonderful things."
- Birch's dialogue: "Why do you call yourselves Treeminders?" (The Lone Wanderer)
"We care for this place and keep it safe from those who would seek to exploit it. He gives to us, so we give back to Him. It's an arrangement that's worked well for almost two decades. We shun technology and embrace nature. That's the life of a Treeminder"
- Birch's dialogue: "How did the Treeminders begin?" (The Lone Wanderer)
"When I first beheld the glory of the Great One, I knew there'd be others who would seek to do Him harm. I also knew He would be calling others that he felt were pure of heart to protect Him. Right then and there I created the Treeminders... the Great One's last line of defense."
- Birch's dialogue: "I think you're all a bunch of lunatics." (The Lone Wanderer)
"Ha ha ha. I don't expect you to understand everything you see here. After all, you carry the taint of technology upon you. Treeminders shun technology and embrace nature. All technology has brought the world is violence and death."
- Birch's dialogue: "Were you aware that Laurel gave me this Liniment?" (The Lone Wanderer)
"Of course I am. I know everything that goes on in Oasis. Laurel is free to pursue whatever path she chooses. A long time ago, it was agreed that the Tree Father and the Leaf Mother hold equal say. I don't seek to change those customs. I just hope that when the time comes, you'll make the right choice."
- Birch's dialogue: "If you hate technology, why carry weapons?" (The Lone Wanderer)
"Sadly, the Wasteland is a hostile place and sometimes we're forced to defend ourselves against it. If that means fighting with manufactured weapons, then so be it. Had He not asked to see you, you never would have gotten this close to the gates."
- Birch's dialogue: "How do the Treeminders normally handle other Outsiders?" (The Lone Wanderer)
"If the Outsider doesn't have His blessing, we simply deny him entry to our home. If they persist on trying to gain entry, or display any hostile act that could potentially harm the Great One, we take action. If the Outsider were a Raider or some other nefarious type, we ensure they don't report our location back to their comrades."
- Birch's dialogue: "Which brings us to why I'm here, I suppose." (The Lone Wanderer)
"Yes. Indeed it does. Sorry I don't speak to Outsiders very often and I tend to get lost in conversation. As you approached Oasis, He said you were coming and I was sent out to meet you personally with a request. He wishes to meet with you. You'd be the first Outsider to do so in a very long time."
"So what was all this about meeting someone?" (The Lone Wanderer)
"Yes, my apologies. I don't speak to Outsiders very often and I tend to get lost in conversation. As you approached Oasis, He said you were coming and I was sent out to meet you personally with a request. He wishes to meet with you. You'd be the first Outsider to do so in a very long time."
- Fallout 3 Official Game Guide Game of the Year Edition p.319: "Oasis is a fertile, verdant dot in the center of all the desolation. This odd hidden vale is home to a strange, tribal-like people who call themselves the Treeminders. You are welcomed into Oasis with open arms, and their leader, Tree Father Birch, invites you to meet their god. Oasis is tucked away inside a giant rocky outcrop in the mountains just northeast of the monorail and freeway skeletons, and the entrance is close to a rope bridge."
- Birch's dialogue: "Amazing, this place seems so abundant with animal life." (The Lone Wanderer)
"The innocent creatures are drawn here by His gifts. They come from across the Wasteland to live in this sanctuary. This part of the world is healing, my friend, and it's all thanks to Him."
- There are sounds of birds, crickets, and squirrels as background sounds in Oasis.
- Yew's dialogue: "Hey, maybe if you tell me what Harold is scared of I won't be scared either." (The Lone Wanderer)
"Tell me, too! Then I won't be scared either." (The Lone Wanderer)
"Awww, I never knew an Outsider could be scared of anything! Harold told me that he's scared of fire. If fire ever got on him, it would burn him and Bob until they were all gone. That's why we keep the fires far away from him."
- Yew's dialogue: "Any information you could give me on the caves below Oasis would be helpful." (The Lone Wanderer)
"My parents told me never to go past the old gate... there are monsters in there!"
- Three Dog's dialogue: "Here's a question for all you faithful listeners. Have you guys and gals ever seen... a tree? No, no, no. Not those shriveled black things. I’m talking real trees. Brown bark, green leaves, photosynthesis, all that good stuff. Now what if I, the all-powerful Three Dog, were to tell you that somewhere right here in the Capital Wasteland, there’s a place with LOTS of trees. A veritable Oasis of green in that depressing sea of brown. Look, it was years ago, and I MAY have been experimenting with Jet at the time, but I’m telling you, it’s out there..."
- The Preacher's dialogue: "..."
- Poplar's dialogue: "Any information you could give me on the caves below Oasis would be helpful." (The Lone Wanderer)
"Be wary of the water in the caves, I fear they still carry the sting of radiation from the bombs."