Dynamic magnetic properties of Ta-O/Co20Fe60B20 bilayer films are strongly influenced by the oxidation condition of the Ta-O layer. The oxidation of the Ta-O layer by a slight amount of oxygen with a pressure (POxygen) of 0.03 Pa decreases in-plane damping constant (αIP), and increases the effective magnetization (4πMs,eff). Then, both αIP and 4πMs,eff maintain their values by increasing POxygen up to 0.3 Pa. The out-of-plane damping constant (αOP) showed a similar tendency to that of αIP against POxygen, although αOP is much smaller than αIP in every POxygen. αOP reaches to 0.0033 for sample oxidized at 0.03 Pa. It was suggested that αIP consists of both the intrinsic damping and the extrinsic damping, while αOP is closer to the intrinsic damping. The control of αOP and αIP by the oxidation would be beneficial in designing the high frequency spintronic devices.