Conventionally, both aeration tanks and bubble columns are supplied oxygen from dispersed bubbles. However, the geometry of the aeration tanks is greatly different from that of bubble columns. Although almost all oxygen is absorbed from dispersed bubbles in the water for bubble columns, the aeration tank has such a large free surface area that the oxygen transfer from atmosphere cannot be neglected. Therefore, it is very important to investigate oxygen transfer from not only the free surface, but also bubbles. In this study, the volumetric oxygen transfer coefficient, gas holdup and its two dimensional profile were measured in a large scaled aeration tank of typical size for industrial sewage treatment. Moreover, the effect of the aeration method on gas holdup and water circulation such as spiral flow aeration and whole floor aeration were also investigated. By analyzing the experimental results, a simultaneous oxygen transfer model which consists of the sum of oxygen transfer coefficients not only between bubbles and water, but also between the atmosphere and the free surface, is proposed. The model was evaluated by comparing with two alternative models for the experimental overall oxygen transfer coefficients. The proposed model estimated the experimental results better than the other models.