An oasis was an area in the middle of a desert that was lush.
In 2151, Jonathan Archer referred to areas where life was concentrated on the rogue planet Dakala as oases. (ENT: "Rogue Planet")
Ezral also described the Kantare supply ship's crash landing location as an oasis. (ENT: "Oasis")
In 2267, Leonard McCoy remarked that the terms "void" and "star desert" made him think of oases, dunes and mirages. (TOS: "The Squire of Gothos ")