Oasis is near the top of the map some where in the middle. I know it's near a vault, I think it's vault number 92 Near the center of the north edge of the map, there is a road running parallel to a railway line. Follow the railway/road until you find two abandoned railway cars. At the northern most railroad car, stand on the eastern side of of the car and face west. You will see the letters "CMR" on the side of the car. Behind you will be a pile of highway debris. Turn around and proceed east over the pile of debris. you will find a bridge over a gorge. Cross the bridge and follow the path in a generally easterly direction. This path leads directly to Oasis.