Chemical and isotopic compositions of magmatic crystals provide important information to distinguish between deep juvenile and crustal contributions. In this work, high-resolution multicollector secondary ion mass spectrometry data reveal strong variations of δ18O values in three plagioclase crystals (800 – 1700 μm) from two representative basaltic andesite samples of the 2010 Merapi eruption (Central Java, Indonesia). The δ18O values (from 4.6 to 7.9 ‰) are interpreted to reflect oxygen isotope heterogeneity in the melt composition during plagioclase growth. The lowest δ18O values (4.6 – 6.6 ‰) are found in anorthite-rich cores (An82–97), whereas higher δ18O values (5.7 to 7.9 ‰) are found in anorthite-poorer zones (An33–86), typically in crystal rims. Combining these new plagioclase δ18O data with δ18O of calc-silicate crustal xenoliths erupted between 1994 – 1998, the composition of glass inclusions hosted by the anorthite-rich plagioclase (An82-92), available experimental data, and the results of thermodynamic modelling using the Magma Chamber Simulator code, we conclude that the abundant anorthite-rich cores crystallized from a mantle-derived hydrous basaltic to basaltic trachyandesite melt that recharged a deeper (200-600 MPa) magma storage zone, whereas lower anorthite zones crystallized at shallower levels (100-200 MPa). The oxygen isotope variations in the plagioclase are explained by a two-stage model of interaction of the hydrous, mafic mantle-derived magma (1) with old crustal rocks depleted in 18O due to high temperature alteration that yielded the low δ18O values in the anorthite-rich cores at deep levels (13 – 20 km), and later (2) with 18O-enriched carbonate material that yielded the high δ18O values in anorthite-poorer zones at shallow levels (∼4.5 – 9 km). Thermodynamic modelling is consistent with ∼18 wt.% assimilation of crustal calc-silicate material at 925 – 950°C and 100 – 200 MPa by the 2010 Merapi basaltic andesite magma prior to eruption. Timescales for plagioclase phenocryst growth and residence in the magmatic plumbing system are ⩽34 years. The combined data thus reveal efficient magma recharge and crustal assimilation processes that characterize the open-system magma storage and transport systems associated with the 2010 Merapi eruption.