An apparatus of controlling oxygen supply for a boiler having wind boxes 2a and 2b arranged on left and right of a boiler body 1 is configured to control supply balance of oxygen by adjusting oxygen supply opening degrees of the left and right oxygen distribution dampers 15a and 15b in accordance with a oxygen supply flow rate and left and right actually measured oxygen concentrations when controlling the oxygen supply amounts and to control the exhaust gas flow rates by correcting an exhaust gas flow rate set value corresponding to a boiler load demand in accordance with an oxygen concentration set value corresponding to a boiler load demand, left and right actually measured exhaust gas flow rates and left and right actually measured oxygen concentrations and adjusting left and right exhaust gas introduction opening degrees of the left and right exhaust gas introduction dampers 8a and 8b when controlling the exhaust gas flow rates.