In situ ion microprobe analyses of oxygen isotopes in Yamato-86009 and Murchison chondrites show that they contain abundant olivine-rich inclusions that have large oxygen-16 (O-16) excesses, similar to those in spinel grains in calcium-aluminium-rich inclusions in Allende and other carbonaceous chondrites. The existence of O-16-enriched olivine-rich inclusions suggests that oxygen isotopic anomalies were more extensive in the early solar system than was previously thought a nd that their origin may be attributed to a nebular chemical process rather than to an unidentified O-16-rich carrier of presolar origin.