Fallout 3 Oasis Location

go all the way north as far as you can go, to the very top of the map, all the way up, almost directly above vault 101. at some point, you should see a huge plateau look around the northwest-ish area of it and eventually you will find a small crevasse that leads to oasis. but trust me, it will probably still take a while to find it exactlt because

A:my directions arent exactly perfect, i know.

and B; its hard to spot the crevasse.

hope i helped.