New oxygen isotope compositions of olivine phenocrysts collected across the Galapagos archipelago show a larger range of δ18Oolivine values than previously reported (4.74–5.40‰) in the region. Olivines from Fernandina, Floreana, and Pinta, which represent the main radiogenic isotope end-members of the Galapagos hotspot, have δ18O values of 5.02 ± 0.08‰ (1σ), within the accepted range of the oxygen isotope composition of mantle olivines. In general, δ18Oolivine values do not correlate with radiogenic isotope compositions of hosting lavas. Instead, the span of δ18Oolivine values is more consistent with widespread lithospheric contamination. δ18Oolivine values at the higher end of the range in the Galapagos are correlated with indices of crustal assimilation including Sr/Sr*. Values below the normal mantle range can be explained by assimilation and fractional crystallization processes. Olivines that have δ18O values below the normal mantle range come from the western part of the archipelago with the thickest lithosphere. This is consistent with melt interacting with crust that underwent hydrothermal alteration at elevated temperature, causing a decrease in δ18O values. In contrast, the highest δ18O values of the Galapagos come from areas underlain by thin lithosphere in the eastern part of the archipelago. This is consistent with shallow crust/melt interaction that is generally associated with δ18O values above the normal mantle range. These results suggest that while the three end-member components of the Galapagos mantle have a generally homogenous δ18O value indistinguishable from “normal” upper mantle, there is a more widespread effect of lithospheric contamination in melts than previously thought.