We investigated the oxygen isotope effect (IE) on the transition temperature Tc in the spin-triplet superconductor Sr2RuO4 (with the intrinsic Tc0=1.5 K). A clear IE shift in Tc was observed. Moreover, we found that the IE coefficient alpha exhibits an unusual variation with Tc. For lower-Tc crystals containing impurities and defects, α is positive and increases with decreasing Tc; α(Tc) is described well by the universal behavior expected theoretically. However, for crystals with Tc approaching Tc0, α deviates from the universal α(Tc) and becomes negative. We discuss its possible mechanism on the basis of existing models.