Breathing need (4)
(Other definitions for lung that I've seen before include "Pulmonary organ" , "breather" , "urban area" , "Air bag?" , "Organ providing oxygen to the blood" .)
(Other definitions for oxygen that I've seen before include "Element forming part of water and air" , "Vital gas element" , "The second most common element on earth" , "Life-supporting gas" , "publicity, perhaps" .)