This study evaluates the buffering of seawater oxygen isotopes at midocean ridges, using a process-based model of hydrothermal circulation and reactive transport of oxygen isotopes. The buffering intensity shown by the model is significantly weaker than previously assumed. Oxygen isotopes of oceanic crust are consistently relatively insensitive to seawater isotopic composition, which explains the ancient oceanic crust without invoking a constant seawater oxygen–isotopic composition through time.