Oxidative coupling of methane with CO₂ (CO₂-OCM) is beneficial because it avoids complete oxidation, an undesired reaction that occurs during O₂-oxidative coupling of methane. Previous literature has sparsely considered the respective roles of CO₂ and O₂ as reactants at CO₂-OCM reaction conditions. This work explores the roles of CO₂ and O₂ in OCM over Sr/La₂O₃ with different reactant oxidant mixtures. While CO₂ alone did not significantly activate methane at 875 °C, a tenfold increase in C₂ yield resulted from adding 2 % O₂. The role of CO₂ in heterogeneous reactions was supported by XPS and XRD, which showed presence of carbonate species on the catalyst surface while the bulk phase remained La₂O₃. The work shows that a small amount of gaseous oxygen in the reactants can greatly improve methane conversion, and while oxygen is crucial in converting methane, CO₂ plays an important role during ethane dehydrogenation.