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Oxygen-isotope effects in the electrical and magnetic properties of epitaxial thin films of La_1-xCa_xMnO₃ (x = 0.2, 0.3, and 0.4) have been studied. The films were grown on [100] LaAlO₃ single crystal substrates by pulsed laser deposition using KrF excimer. We find that the oxygen-isotope shift of the Curie temperature decreases significantly with increasing hole concentration. The activation energies of the Hall coefficient and resistivity are slightly different and both depend on the oxygen-isotope mass. The field-dependent data of the Hall Effect and electrical conductivity for the oxygen-isotope-exchanged films clearly demonstrate that the colossal magnetoresistance effect and the oxygen-isotope effects in doped manganites cannot simply be explained by the double-exchange model along with strong lattice polaronic effects. The results could be well explained if the spins of polarons would be quenched by formation of clusters of polaronic pairs.