The structural and electronic properties of a series of SrMnO3 thin films grown on different substrates leading to different biaxial strains are examined by means of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS). Here, we report a comprehensive theoretical investigation on the impact of oxygen nonstoichiometry and strain on the O-K and Mn-L-2,L-3 electron energy-loss near-edge structures of SrMnO3 with the aid of the all-electron density functional theory code WIEN2K. Our experimental and theoretical EELS data reveal pronounced peak-height changes and energy shifts as a function of strain, which are ascribed to differences in the oxygen vacancy density rather than to a direct effect of lattice distortions. In addition, we find that the formation of oxygen vacancies in SrMnO3 is likely to be enhanced by tensile strain. Compared to the unstrained material, the vacancy formation energy is reduced by 0.25 eV under 3.78% tensile strain.