Reactions of d(0) amides M(NMe2)(4) (M = Zr, 1; Hf, 2) with O-2 have been found to yield unusual trinuclear oxo aminoxide complexes M-3(NMe2)(6)(mu-NMe2)(3)(mu(3)-O)(mu(3)-ONMe2) (M = Zr, 3; Hf, 4) in high yields. Tetramethylhydrazine Me2N-NMe2 was also observed in the reaction mixtures. Crystal structures of 3 and 4 have been determined. Density functional theory calculations have been performed to explore the mechanistic pathways in the reactions of model complexes Zr(NR2)(4) (R = H, 5; Me, 1) and [Zr(NR2)(4)](2) (R H, 5a; Me, 1a) with triplet O-2. Monomeric and dimeric reaction pathways in the formation of the Zr complex 3 are proposed.