Emphasis is put on a precise knowledge of the oxygen isotope shift exponent αO is the so far synthesized high-Tc superconductors in order to elucidate the basic mechanism. The known data, see αa in Table 6, indicate the presence of a presumably anharmonic coupling of pyramidal apex oxygen motion along the c-axis in these highly in-plane correlated superconductors. αa is computed from αO=rαa, based on the conjecture that the ratio r of apex to total oxygen-ion content per unit cell is relevant. The related experimental and theoretical literature is reviewed. © 1990 Springer-Verlag.