Man, oases are such an interesting topic and more relevant than ever with fresh water being a more and more serious issue almost everywhere. Leaving the Wikipedia page and a short snippet here for anyone who's curious:

Oases develop in "hydrologically favored" locations that have attributes such as a high water table, seasonal lakes, or blockaded wadis. Oases are made when sources of freshwater, such as springs, underground rivers, or aquifers irrigate the surface naturally or via man-made wells.

Another interesting thing is the microclimate created by oases and how these have been managed and cultivated for centuries by people working and living in them. Some oases (like al-Hasa in Saudi-Arabia) are huge.