The isotopic composition of dissolved oxygen in the mesopelagic ocean is a unique tracer of respiration and transport. New ? 18O of O2 data from the tropical South Atlantic oxygen minimum zone are presented and compared to global ? 18O data. The ? 18O variability in oxygen poor waters is attributed to differences in physical and biogeochemical processes. Simple respiration-transport models show that both isopycnal diffusion and advection must be properly considered when interpreting oxygen isotope signatures along an isopycnal surface. We estimate rates of respiration and oxygen isotope fractionation for the study region using a two-dimensional (2-D) isopycnal and 1-D diapycnal model. Estimated respiration rates are consistent with previous studies. However, to account for observed ? 18O values at low [O2], model solutions need to invoke either very low [O2] that have not been observed in the South Atlantic or an isotope effect that is lower than values measured in the laboratory or euphotic zone.