In this paper, we present a thorough study of the electronic structures and binding energies of O₂ to iron and manganese porphyrins (FeP and MnP), employing a state-of-the-art computational technique known as second-order perturbation theory based on density matrix renormalization group (DMRG-CASPT2). By investigating an extensive list of different binding modes and spin states, we provide a clear and conclusive description of the ground state of MnP–O₂, confirming available experimental evidences. Our results show that MnP–O₂ favours a side-on quartet structure, with strong charge transfer between MnP and O₂. We also calculated the standard binding enthalpies of O₂ to different metal porphyrins and showed that an agreement between calculated results and experimental data to within 2 kcal mol⁻¹ can be achieved. Our calculations confirm the experimental observation that the binding of O₂ to manganese porphyrin is stronger by around 4–6 kcal mol⁻¹ than to the corresponding ferrous porphyrin.