ABO_3−δ perovskites are utilized in many applications including optical gas sensing for energy systems. Understanding the opto-electronic properties allows rational selection of the perovskite-based sensors from a diverse family of ABO_3−δ perovskites, associated with the choices of A and B cations and range of oxygen concentrations. Herein, we assess the impact of oxygen vacancies on the electronic structure and optical response of pristine and oxygen-vacant ABO_3−δ (A = La, Sr; B = Cr, Mn) perovskites via first-principles calculations. The endothermic formation energy for oxygen vacancies shows that the generation of ABO_3−δ defect structures is thermodynamically possible. LaCrO₃ and LaMnO₃ have direct and indirect ground-state band gaps, respectively, whereas SrCrO₃ and SrMnO₃ are metallic. In the presence of an oxygen mono-vacancy, however, the band gap decreases in LaCrO_3−δ and vanishes in LaMnO_3−δ. In contrast to the decrease in the band gaps, the oxygen vacancies in ABO_3−δ are found to increase optical absorption in the visible to near-infrared wavelength regime, and thus lower the onset energy of absorption compared with the pristine materials. Our assessments emphasize the role of the oxygen vacancy, or other possible oxygen non-stoichiometry defects, in perovskite oxides with respect to the opto-electronic performance parameters that are of interest for optical gas sensors for energy generation process environments.