The ¹⁸O/¹⁷O ratio of the solar system is 5.2 while that of the interstellar medium (ISM) and young stellar objects is ~4. This difference cannot be explained by pollution of the Sun's natal molecular cloud by ¹⁸O-rich supernova ejecta because (1) the necessary B-star progenitors live longer than the duration of star formation in molecular clouds, (2) the delivery of ejecta gas is too inefficient and the amount of dust in supernova ejecta is too small compared to the required pollution (2% of total mass or ~20% of oxygen), and (3) the predicted amounts of concomitant short-lived radionuclides (SLRs) conflicts with the abundances of ²⁶Al and ⁴¹Ca in the early solar system. Proposals for the introduction of ¹⁸O-rich material must also be consistent with any explanation for the origin of the observed slope-one relationship between ¹⁷O/¹⁶O and ¹⁸O/¹⁶O in the high-temperature components of primitive meteorites. The difference in ¹⁸O/¹⁷O ratios can be explained by enrichment of the ISM by the ¹⁷O-rich winds of asymptotic giant branch (AGB) stars, the sequestration of comparatively ¹⁸O-rich gas from star-forming regions into long-lived, low-mass stars, and a monotonic decrease in the ¹⁸O/¹⁷O ratio of interstellar gas. At plausible rates of star formation and gas infall, Galactic chemical evolution does not follow a slope-one line in a three-isotope plot, but instead moves along a steeper trajectory toward an ¹⁷O-rich state. Evolution of the ISM and star-forming gas by AGB winds also explains the difference in the carbon isotope ratios of the solar system and ISM.