The magnitude of the oxygen isotope eff'ect in the high-T, superconductor YBa2Cu3O7 has been determined by substitution of ' O for ' O. A series of cross exchanges was performed on highquality polycrystalline specimens to eliminate uncertainties due to sample heat treatments and sample inhomogeneities.Magnetic measurements suggest a relative isotope shift for the bulk material of -0.17~0.03 K at 80%%uo ' 0 substitution, yielding ab"~k=0.019+ 0.004 where T, -M and M is the oxygen mass. Resistivity measurements in freshly prepared specimens reveal filamentary superconductivity 1 or 2 K above the bulk superconducting transition temperature. The isotope shift associated with the filamentary superconductivity is similar to but slightly larger than the bulk shift: af;~=0.028+ 0.003. The filamentary superconductivity is time dependent and disappears several months after sample preparation.We show that the above values of a are inconsistent with the standard three-dimensional phonon-mediated Bardeen-Cooper-Schriefer theory, and discuss implications for alternative possibilities.