In this report we present a study on oxygen transport in c-axis-oriented high-Tc superconducting YBa2Cu3O7-δ thin films. We have performed oxygen outdiffusion experiments, by gently heating these films in an ultrahigh-vacuum system. A rotating-analyzer ellipsometer has been used as an in situ monitor. For the oxygen transport we have developed an optical model. Our in situ ellipsometric measurements cannot be described without the assumption that short circuits are present in the films. The oxygen diffuses in the a-b planes of the YBa2Cu3O7-δ to the short circuits. Along these short circuits, the oxygen leaves the films. In our case, the average spacing between the short circuits is 2.5±0.2 μm. No outward diffusion of oxygen occurs along the c axis in the YBa2Cu3O7-δ films.