Aerobic oxidation of metallaborane compounds is an unexplored field apart from the few reports on accidental oxidation leading to oxametallaboranes. An effective method for the synthesis of group 5 oxametallaboranes has been developed by the oxidation of [(CpM) 2 (B 2 H 6 ) 2 ] (M = Ta/Nb) (Cp∗ = η 5 -C 5 Me 5 ). The reaction of [(CpM) 2 (B 2 H 6 ) 2 ] (M = Ta/Nb) with O 2 gas at room temperature yielded oxametallaboranes [(CpM) 2 (B 4 H 10 O)] (for 1, M = Nb; for 2, M = Ta). Density functional theory calculations signify an increase in the HOMO-LUMO energy gap for 1 and 2 as compared to that for the parent metallaboranes, [(CpM) 2 (B 2 H 6 ) 2 ] (M = Ta/Nb). Reaction of 1 and 2 with [Ru 3 (CO) 12 ] led to the isolation of fused metallaborane clusters [(CpNb) 2 (B 2 H 4 O){Ru(CO) 2 } 2 (B 2 H 4 ){Ru(CO) 3 } 2 {μ-H} 4 ] (3) and [(CpTa) 2 (B 3 H 4 O){Ru(CO) 2 } 3 {μ 7 -B}{μ-CO} 2 {μ-H} 4 ] (4). The structure of 3 may be considered as a fusion of five subunits [two tetrahedra (Td), two square pyramids (sqp), and one trigonal bipyramid (tbp)]. One of the key features of cluster 4 is the presence of a μ 7 -boride atom that shares three cluster units (one monocapped trigonal prism and two Td). All the compounds have been characterized by mass spectrometry, infrared spectroscopy, and 1 H, 13 C, and 11 B nuclear magnetic resonance spectroscopy, and the structural types were unequivocally established by crystallographic analysis of compounds 1, 3, and 4. © Copyright 2018 American Chemical Society.