Ocean oasis (4)
I believe the answer is:
isle
'ocean oasis' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for isle that I've seen before include "Eg, Staffa" , "one ringed in the main?" , "eg Man, Dogs" , "Piece of land in water" , "---- of Dogs, Thames peninsula" .)