The variation of oxygen content in the epitaxial La0.8Sr0.2MnO3-delta films of 12-75 nm thick onto NdGaO3 substrates was quantitatively investigated by indirect method during heat treatment procedure in air at temperature range 873-1073 K. The experimental time dependences of oxygen content variation were investigated theoretically with taking into account the effect of inhomogeneous diffusion of oxygen over the films thickness to provide optimal choice of temperature and duration of the heat treatment procedure. In films 25-75 nm thick, the effect of "stabilization" of the oxygen content (OC = 3) is revealed, which indicates the presence of an energy barrier that prevents the incorporation of an excess oxygen atom into the near-surface layer of the film at optimal conditions of heat treatment. Keywords: epitaxial films, strain, oxygen index, non-uniform oxygen diffusion.