Lavas from the Samoan volcanic chain show the most enriched geochemical signatures ever documented in oceanic lavas (87Sr/86Sr as high as 0.7205). In order to test the hypothesis that their source contains a component of recycled upper continental crust, we measured oxygen isotope compositions of olivine phenocrysts from these lavas. Correlations between δ18O of olivines (5.11‰–5.70‰) and 87Sr/86Sr and 207Pb/204Pb of whole rocks, as well as Ce/Pb and Nb/Th ratios of whole rocks, indicate that (1) measured δ18O are primary, mantle-derived values, and (2) the enriched mantle source of these lavas contains continental crust or its derivative sediments. The observed trend between δ18O and 87Sr/86Sr can be fit using either clastic marine sediment or continental crust values of δ18O, Sr concentration, and 87Sr/86Sr, but only those for clastic marine sediments are compatible with trace element modeling. We conclude that the enriched source for Samoan basalts was created by sedimentation of continent-derived material into a marine environment, followed by subduction and mixing with ambient mantle.