Alloyed triple A-cation perovskites containing a mixture of Cs, methylammonium (MA) and formamidinium (FA) cations are attracting intense attention because of their high photovoltaic performance and relative stability. However, there is limited fundamental understanding of their vacancy defect behaviour and influence of molecular oxygen on their electronic and stability properties. In this combined computational-experimental study, we investigate the (FA,MA,Cs)Pb(I,Br)3 model system with its simulated atomistic structure presented for the first time and supported by X-ray diffraction data. We examine how iodide vacancies and O2 molecules influence the local geometry and electronic structure. Our calculations, supported by Kelvin Probe contact potential difference and photoluminescence measurements, show that introduction of O2 leads to a p-doped triple-cation perovskite, and passivates iodide vacancies resulting in enhanced luminescence efficiency. These results have important implications for the performance and stability of mixed-cation perovskites in optoelectronic devices.