What the hell is Oasis?

see you don't even know. Even though it's a quest in the game with a trophy.

It's the one where you kill the tree man.

I just realized that you werent the person advocating Fallout3 in the first place.
Long story short; it's an area in a desolate part of the map you would never really think to check. On top of that, if the arrow does show up on your mini-map alerting you of it's presence, there is only one way in and it's not very intuitive.

It's simply soemthing you would never think of checking unless you walked across the map back and forth for hours upon hours upon hours.

RE5

I dont have Fallout 3 but i saw trophy list

See my post below that one.

Would it be really cruel joke if I said Wipeout HD ? :)