The whole-rock oxygen isotopic composition of the main units of the Sudbury Irruptive and surrounding rocks has been studied, using samples from two traverses of the North Range and one of the South Range. Norite has an average δ18O of 6.7‰, about 1‰ greater than that of fresh oceanic basalts, and similar to that of some continental basalts. Granophyre is slightly richer in 18O (δ18O = 7.3‰). In neither unit is there significant correlation between δ18O and SiO2 content or degree of alteration as estimated by water content or microscopic appearance. The pervasive hydrous alteration of norite and granophyre apparently occurred in the presence of a small volume of water whose isotopic composition was buffered by the igneous rocks. Inclusion-free devitrified glass ("melt rock") and matrix from the Onaping Formation have δ18O values in the range 6.2 – 12.1‰. Its average δ18O is 8.2‰, comparable to that of Archean gneiss [Formula: see text], as required by the model for the origin of the formation as a fallback breccia from a meteoritic impact. However, the chemical composition of the Onaping rocks requires an admixture of rocks much more mafic than typical Archean gneiss (e.g., greywackes or gabbros of the Southern Province). Norites of the South Range are 1‰ heavier than those of the North Range, possibly due to assimilation of 18O-rich rocks of the McKim Formation. It has been suggested that the granophyre was produced through assimilation of Onaping rocks by the norite; this is consistent with the oxygen isotopic composition of the three rock types but not with their chemical compositions, which show the granophyre to be more depleted in MgO than the Onaping Formation. The granophyre's chemical composition is consistent with an origin by differentiation from a magma with a composition equivalent to that of the transition (oxide-rich) gabbro.