We describe the quenching behavior of thermally activated delayed fluorescence (TADF) from 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) by O-2 in bis(2-ethylhexypsebacate (DOS). Importantly, the high quantum yield of photoluminescence (phi(PL) = 0.93) with efficient TADF was dramatically decreased by O-2. The quenching rate constants of the S-1 and T-1 states were estimated to be in the order of 10(9) and 10(8) s(-1)M(-1), respectively. A DOS-plasticized polystyrene film containing 4CzIPN was prepared for optical O-2 sensing to determine O-2 concentration.