The molecular and electronic structures of a series of small boron monoxide and dioxide clusters BnOm (n = 5-10, m = 1, 2) plus their anions were predicted. The enthalpies of formation (Delta H-f's), electron affinities (EAs), vertical detachment energies, and energies of different fragmentation processes are predicted using the G3B3 method. The G3B3 results were benchmarked with respect to more accurate CCSD(T)/CBS values for n = 1-4 with average deviations of +/-1.5 kcal/mol for Delta H-f's and +/-0.03 eV for EAs. The results extend previous observations oil the growth mechanism for boron oxide clusters: (i) The low spin electronic state is consistently favored. (ii) The most stable structure of a neutral boron monoxide BnO is obtained either by condensing 0 on a BB edge of a B-n cycle, or by binding one BO group to a Bn-1 ring. The balance between both factors is dependent oil the inherent Stability of the boron cycles. (iii) A boron dioxide is formed by incorporating the second O atom into the corresponding monoxide to form BO bonds. (iv) A BnOm- anion is constructed with BO groups bound to the Bn-1 or Bn-2 rings (yielding the Bn-2(BO)(2)(-) species). This becomes the preferred geometry for the larger boron dioxides, even in the neutral state. The boronyl group mainly behaves as an electron-withdrawing substituent reducing the binding energy and resonance energy of the oxides. (v) The boron oxides conserve some of the properties of the parent boron clusters such as the planarity and multiple aromaticity.