Oases are a terrain feature in Civilization VI. Oases are found on flat inland Desert tiles.
Oasis tiles have a base yield of +3 
Food and +1 
Gold. The Lady of the Reeds and Marshes pantheon increases the yield of Oasis tiles by an additional +1 
Production. Like Lake tiles, Oases provide fresh water to adjacent tiles, though there is no movement penalty for embarking/disembarking on an Oasis.
Oasis tiles cannot be developed with tile improvements, districts, or wonders. Cities may not be founded on Oasis tiles.
Strategy
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. 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).
Oases do benefit from the effect of the Petra Wonder, and becomes a quite accomplished tile.
Civilopedia entry
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.
Gallery
.png "Oasis tile in-game (Civ6).png (633 KB)")