The ratio of 18O to 16O in water of the world oceans is examined using the GISS ocean general circulation model. This ratio is governed by fractionation during evaporation and sea-ice formation and by the isotopic content of precipitation and runoff entering the oceans. The δ18Oω at any point depends on a balance between local evaporation, precipitation, and regional ocean and sea-ice processes. Surface δ18Oω equilibrates after 30–50 model years to produce a pattern of high values (0.5–1.5 ‰ ) in the sub-tropics (highest in the Indian and Atlantic oceans) and low values (−15-−1‰ ) in the mid-to-high latitudes. Arctic values are low especially where runoff enters from Arctic rivers. Comparisons with data show that regional linear relationships between δ18Ow and salinity are reasonably captured by the model. Surface data from the entire ocean show a clear distinction in slope between the tropical oceans and the extra-tropics, however, departures from the linear relationships are significant. Importantly for attempts at calculating paleo-salinities, the seasonal and longer-term temporal gradients (Δδ18Ow/ΔS at a fixed point) are not generally equal to the spatial gradient (at a fixed time).