If the ability of Cu5 atomic quantum clusters (AQCs) to create states within the bulk gap of TiO2 persists in the presence of oxygen, then the absorption spectrum of the AQC-TiO2 complex could be matched to the peak of the solar spectrum, thereby enhancing its photocatalytic activity under ambient conditions. Here we demonstrate that this is indeed the case, by examining the electronic structure of Cu5 AQCs adsorbed on a rutile (110) TiO2 surface in the presence of oxygen. We find that adsorbed oxygen changes the most stable AQC isomer from a 2-dimensional trapezoidal structure to a 3-dimensional bipyramidal structure. Furthermore, the bipyramidal AQC creates two ferromagnetically coupled polarons in the TiO2. In contrast, the trapezoidal AQC only creates a single polaron. In the presence of oxygen, these polaronic states associated with the bare cluster are joined by further mid-gap energy levels, formed from hybrid states located on copper and oxygen atoms. Dissociated oxygen-dimer adsorption on Cu5 generally leads to significant deformation of the whole cluster, which accounts for the dramatic drop of the total energy compared to molecular oxygen dimer adsorption. The ability of Cu5 AQCs to create states within the bulk gap of TiO2 under ambient conditions demonstrates that the absorption spectrum of the AQC-TiO2 complex is more closely matched to the peak of the solar spectrum, than either the AQC or TiO2 alone.