Abstract– High-precision isotope imaging analyses of reversely zoned melilite crystals in the gehlenitic mantle of Type A CAI ON01 of the Allende carbonaceous chondrite reveal that there are four types of oxygen isotopic distributions within melilite single crystals: (1) uniform depletion of 16O (δ18O ≈ −10‰), (2) uniform enrichment of 16O (δ18O ≈ −40‰), (3) variations in isotopic composition from 16O-poor core to 16O-rich rim (δ18O ≈ −10‰ to −30‰, −20‰ to −45‰, and −10‰ to −35‰) with decreasing åkermanite content, and (4) 16O-poor composition (δ18O ≥ −10‰) along the crystal rim. Hibonite, spinel, and perovskite grains are 16O-rich (δ18O ≈ −45‰), and adjoin 16O-poor melilites. Gas-solid or gas-melt isotope exchange in the nebula is inconsistent with both the distinct oxygen isotopic compositions among the minerals and the reverse zoning of melilite. Fluid-rock interaction on the parent body resulted in 16O-poor compositions of limited areas near holes, cracks, or secondary phases, such as anorthite or grossular. We conclude that reversely zoned melilites mostly preserve the primary oxygen isotopic composition of either 16O-enriched or 16O-depleted gas from which they were condensed. The correlation between oxygen isotopic composition and åkermanite content may indicate that oxygen isotopes of the solar nebula gas changed from 16O-poor to 16O-rich during melilite crystal growth. We suggest that the radial excursions of the inner edge of the protoplanetary disk gas simultaneously resulted in both the reverse zoning and oxygen isotopic variation of melilite, due to mixing of 16O-poor disk gas and 16O-rich coronal gas. Gas condensates aggregated to form the gehlenite mantle of the Type A CAI ON01.