The effect of O-16 --> O-18 isotope substitution on electrical resistivity and magnetoresistance, was studied for La0.35Pr0.35Ca0.3MnO3 epitaxial thin films deposited onto LaAlO3 and SrTiO3 substrates. For the films on LaAlO3, the isotope substitution resulted in the reversible transition from a metal-like to insulating state. The applied magnetic field (H greater than or equal to 2 T) transformed the sample with O-18 back to the metallic state. The films on SrTiO3 remained metallic at low temperatures for both O-16 and O-18, but the shift of the resistivity peak corresponding to onset of metallic state exceeded 50 K at 3 T after O-16 --> O-18 substitution. This drastic difference in transport properties can be related to the substrate-induced strains having opposite signs for the films on LaAlO3 and SrTiO3. The temperature dependence of resistivity was characterized by hysteresis, especially pronounced in the case of the films on LaAlO3. Such a behavior gives certain indications of the phase separation characteristic of interplay between ferromagnetism and charge ordering.