Scanning tunneling microscopy studies of Cu islands grown on TiO2(110) demonstrate that these islands disappear from the STM images after exposure to oxygen gas (60-2000 Langmuir). Based on X-ray photoelectron spectroscopy experiments, the disappearance of the Cu islands cannot be explained by the loss of Cu from the surface or by a dramatic change in the electronic properties of the islands. The adsorption of oxygen appears to weaken the Cu-Cu bond, allowing two-dimensional (213) islands to form on the surface at the expense of the existing three-dimensional (3D) islands. Thermodynamically, the conversion of 3D to 2D islands is favored by the lower surface free energy of the oxidized Cu compared to that of copper itself. This effect has also been observed for Ni islands, but the rate of island disappearance is slower even though Ni is more easily oxidized than Cu.