The oxygen isotope effect on the first and second order Raman scattering of high-quality La_2-xSr_xCu^16,18O₄ (80% substitution ¹⁶O by ¹⁸O) polycrystalline compounds with x=0.00, 0.015 has been investigated. Measurements were obtained on microcrystallites in the approximate (zz) and (xx/yy) scattering configuration, at low temperatures (down to 10K). The soft mode that appears in the first order zz spectra related with the orthorhombic-to-tetragonal phase transition shows a deviation from the harmonic law at all temperatures studied. The apex oxygen mode follows quite well the mass harmonic law, even at low temperatures. A broad band of apparent B_1g symmetry in the region of 300-330 cm^-1 of the (xx) polarization spectra seems to consist of two modes, one of which shifts with the isotopic substitution and the other is not shifted and therefore is due to La atoms vibrations. Concerning the second order Raman scattering several weak and strong peaks have been observed and categorized to those that involve vibrations of the oxygen atoms or the heavy elements.

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