Oases are a handy terrain feature in the desert, ensuring both food and fresh water even in the absence of Floodplains (Oases are usually found far from Rivers in the desert). Feel free to found a City near it, as an alternative to founding it near a River. Note, however, that the Oasis is in fact a type of a mini-lake, so you cannot place anything on it - not a District, nor a Wonder or Tile Improvement. On the other hand, its shallow waters don't even hinder normal movement (unlike most other terrain features).
Also, since an Oasis is found literally in the middle of the Desert, you will have to think well whether it is really worth it to found a city near it! Unless there are Hills, resources, rivers or other terrain types nearby, a city there will have no way to sustain itself or produce anything! Unless, of course, you manage to construct Petra, which will convert the entire desert nearby into fertile and productive land. The Oasis tile itself will also benefit from the effect of Wonder, and becomes a quite accomplished tile.
The stuff of legends, and vital for those wandering about the desert, oases sit atop a source of clean and plentiful water. Tales have been told of these wonderful outposts in an otherwise barren and lifeless land ... and battles fought to control them.