Not very good doctor about to introduce oxygen (8)
I believe the answer is:
mediocre
'not very good' is the definition.
(mediocre means of middling quality)
I don't understand the rest of the clue.
(Other definitions for mediocre that I've seen before include "Second-rate, undistinguished" , "Fair, middle of the road" , "Rather poor" , "Come ride (anag.)" , "Rather inferior" .)