Hybrid organic-inorganic perovskites have emerged as very successful optically active materials due to their unique electronic and chemical properties. Experiments have shown that oxygen undermines perovskite chemical stability, but enhances charge carrier lifetimes. Focusing on CH3NH3PbI3, which has become the classic material, we demonstrate how and why charge carrier lifetimes change in the presence of oxygen, by carrying out nonadiabatic molecular dynamics simulations combined with time-domain ab initio density functional theory. Calculations have shown that superoxide and peroxide are the common forms of oxygen interacting with CH3NH3PbI3 and that oxygen most readily interacts with iodine vacancies on the perovskite surface. We establish that the iodine vacancy decreases charge carrier lifetimes, because it localizes both electrons and holes, increasing their overlap. By passivating the vacancy, the oxygen species separate electrons and holes and increase the lifetimes by more than an order of magnitude. Passivating the vacancy by water and Lewis bases, such as pyridine and thiophene, also leads to electron-hole separation. The energy gap changes only by a few percent; however, the nonadiabatic coupling becomes much weaker, and the quantum coherence time decreases significantly. The detailed time-domain atomistic analysis of the excited state dynamics rationalizes why the photogenerated charge carriers in perovskites are robust to defects and interactions with chemical species present in air, such as water and oxygen, even though they undermine perovskite chemical stability. The results can apply to other solar energy materials, which are exposed to atmospheric gases and the performance of which often depends on such exposure.