The oxygen isotope ratios for 127 rocks and coexisting minerals from Paleozoic granitoids and clastic metasedimentary rocks of southwestern Nova Scotia have been measured. The whole-rock δ18O values for samples of the South Mountain batholith range from 10.1–12.0‰.But discrete granitoid plutons, located to the south of the South Mountain batholith, have lower δ18O values (7.8–10.4‰). Coexisting minerals from the Nova Scotia granitoids are near isotopic equilibrium, indicating that the whole-rock δ18O values primarily reflect the δ18O of the magma, rather than secondary alteration processes. The Meguma Group clastic metasedimentary rocks that host the Nova Scotia granitoids range in δ18O from 10.1–12.9‰. These clastic metasedimentary rocks show no systematic geographic variation in δ18O. The greenschist facies Meguma Group rocks that host the South Mountain batholith have similar δ18O values to the amphibolite facies equivalents located about the southern discrete plutons. Large scale isotopic exchange between the Meguma Group and the South Mountain batholith, or the southern plutons, is not evident.The relatively high δ18O values of the peraluminous South Mountain batholith (10.1–12.0‰) indicate that it formed by anatexis of 18O-rich clastic metasedimentary rocks. The southern plutons were also derived by partial melting of clastic metasedimentary rocks, but their lower δ18O values reflect exchange of the source material with a low 18O reservoir (mafic magmas?) prior to, or during anatexis.The sheared Brenton pluton is much lower in δ18O (5.0‰) than any of the other rocks, probably because of exchange with low 18O fluids during shearing.