The oxygen isotopic composition of water varies with temperature and precipitation/evaporation cycles, and 18O is enriched in mother's milk compared to drinking water. Oxygen isotope values in teeth therefore help paleoecologists reconstruct environmental changes, and bioarcheologists probe the timing of human weaning during prehistory. Tooth enamel is typically sampled by drilling to recover oxygen inputs from food and water consumed during tooth formation, as well as inspired air. This method has limited spatial resolution, yielding samples that integrate long formation times of unknown chronological age. Here were employ a Sensitive High Resolution Ion MicroProbe (SHRIMP SI) to measure oxygen isotope compositions (δ18O) on a spatial scale of 15–30 μm. Thin sections of wild orangutan, captive macaque, and captive sheep molars were sampled sequentially along the enamel-dentine junction using secondary-ion mass spectrometry. Standardized δ18O values were related to temporal records of formation as well as calcium-normalized barium (Ba/Ca) values. We find that δ18O values measured by SHRIMP SI are nearly identical to those from silver phosphate microprecipitation of diced enamel from a sheep subject to an experimental water switch, confirming the fidelity of this approach for phosphate oxygen recovery. δ18O values increased by 3‰ during a period of exclusive nursing in a macaque infant, consistent with reports of increases during human nursing. Finally we found that orangutan δ18O values rose and fell in parallel with Ba/Ca values, adding further support to the hypothesis that orangutan nursing relates to fluctuating resource availability. This approach may be extended to well-preserved fossil material.