The whole-rock O-isotopic compositions of volcanic and volcaniclastic samples from the Toa Baja drill hole demostrate that low-temperature (<200 ¿C) processes have strongly enriched the island arc materials in 18O. Subsequent to eruption, processes such as subaerial weathering, alteration during transport and deposition in volcaniclastic aprons, submarine weathering, burial diagenesis, and prograde regional metamorphism through the beginning of the prehnite-pumpellyite facies have raised average whole-rock Δ18O values by ~4% for basalt and andesite lava flows, and by ~8% for volcaniclastic sandstones. These O-isotopic disturbances were probably caused by oxygen exchange with regionally circulating seawater under rather high water/rock conditions. The processes associated with ''ageing'' of volcanic and volcaniclastic materials in the oceanic environment are probably more important to the global budgets of the oxygen isotopes than has been assumed in the past. Integration of these results into global models for the oxygen isotopes analysis of more varied oceanic terranes, to determine the generality of the O-isotopic conclusions proferred here, and to more carefully evaluate the relative sizes of volcanic, volcaniclastic, and oceanic oxygen reservoirs and their variabilities in time. ¿American Geophysical Union 1991