Glassy rocks from compositionally zoned ash-flow sheets and cogenetic lava flows, erupted from the late Miocene Timber Mountain–Oasis Valley caldera complex, show systematic oxygen-isotope variations (90 analyses) among all major phenocryst phases: quartz, sanidine, plagioclase, biotite, augite, and magnetite. Oxygen-isotope compositions of each phenocryst phase become lighter in O18 with decreasing age and are interpreted as indicating major interaction between meteoric ground water and batholithic-sized bodies of silicic magma prior to eruption. Each of the ash-flow sheets is compositionally zoned from silicic and phenocryst-poor at the base of more mafic and phenocryst-rich at the top; this pattern represents in inverse order the original compositional zonation in the magma chamber. The changing isotopic fractionations between phenocrysts also reflect crystallization temperatures that varied vertically over several kilometers in the differentiated magma. Posteruption oxidation and cooling effects have not significantly modified oxygen-isotope compositions of phenocrysts from glassy rocks.