IN CONVENTIONAL superconductors, the large dependence of the superconducting transition temperature (T-c) on the isotope mass (T-c proportional to m(-alpha), with alpha approximate to 0.5) gives strong evidence for electron pairing by a phonon-mediated mechanism. The copper oxide superconductors, on the other hand, exhibit a small oxygen isotope effect (alpha approximate to 0.02; see, for example, refs 1-14), suggesting a mechanism more complex than simple phonon-mediated pairing(15). To obtain further insight into this mechanism, it is important to determine the contribution from different oxygen sites in the crystal lattice to the total oxygen isotope effect. Recently, a negative isotope effect associated with oxygen atoms in the copper oxide planes of the crystal structure was reported(13) for highly doped YBa2Cu3O6+x, implying that the apical/chain oxygens make a substantial contribution to the total oxygen isotope shift. Here we investigate this effect in optimally doped YBa2Cu3O6+x, by measuring separately the contributions to the total isotope shift of the planar and apical/chain oxygen sites. The sum of the site-selective isotope shifts equals that of the total measured shift, illustrating the self-consistency of our results. In contrast to the previous Work(13), we find that more than 80% of the total (positive) isotope effect is associated with the copper oxide planes.