The most common and most likely answer to this crossword clue is the 8 letter word WATERING
I've seen this crossword clue in The Daily Telegraph
W
A
T
E
R
I
N
G
Buddy explains!

The straight part
We know that "Oasis" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oasis = watering.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WATERING mean?
WATERING verb- To pour water into the soil surrounding (plants).
- To wet or supply with water; to moisten; to overflow with water; to irrigate.
- To provide (animals) with water for drinking.
- To get or take in water.
- To urinate onto.
- To dilute.
- To overvalue (securities), especially through deceptive accounting.
- To fill with or secrete water.
- To wet and calender, as cloth, so as to impart to it a lustrous appearance in wavy lines; to diversify with wavelike lines.
- An act of watering, i.e. pouring water on something, or diluting a liquid.
- The art or process of giving a wavy, ornamental appearance to something.