Oxygen-isotope compositions are reported for glass inclusions, their host olivines (Fo 8 7 - 9 0 ) and tholeiitic pillow rim glasses from the neovolcanic rift zone of Iceland. The inclusion compositions range from enriched ([La/Sm] n =1.4-4.8) to very depleted ([La/Sm] n =0.08-0.35). The whole rocks and matrix glasses display little variation ([La/Sm] n =0.41-0.51). Glass inclusions vary somewhat stronger in 1 8 O/ 1 6 O ratios (δ 1 8 O=4.0-6.2+/-0.6%%) than the matrix glasses (δ 1 8 O=4.6-5.6+/-0.6%%). The range of olivine phenocryst composition is even wider (δ 1 8 O=2.4-6.9+/-0.5%%), and in some cases the variations within individual crystals reach 1.0-2.5%%. Most glass inclusions are in O-isotopic equilibrium with the surrounding host olivine but not with the central parts of the host crystals; similarly, most olivines are out of equilibrium with their host matrix glasses. This implies that the glass inclusions and olivine phenocrysts can preserve O-isotopic compositions inherited from contrasting parental mantle-derived magmas, which might have mixed in a common volcanic system. The isotopic heterogeneity of individual olivine crystals allows one to estimate their residence times in the magma reservoir using the rate of 1 8 O diffusion in olivine. The time required to achieve isotopic homogeneity over 90-520 μm ranges from 25 to 1030 yr. The δ 1 8 O values found in depleted glass inclusions and their host olivines are unlikely to have resulted from magma contamination by upper crustal rocks. We suggest two possibilities, implying either an O-isotope heterogeneity of the Icelandic mantle or a mixing of two end-members: a refractory component with 'normal-mantle' or somewhat elevated δ 1 8 O values, high Ca/Na and strong light rare earth element depletion mixed with a component having lower δ 1 8 O and Ca/Na but also depleted in incompatible elements. Based on δ 1 8 O-La/Sm and δ 1 8 O-δ 1 1 B relationships, the variations of δ 1 8 O between 4.6 and 6.2%% in enriched and depleted glass inclusions are interpreted to reflect those of primary magmas and, consequently, of their source region.