The lowest-energy structures of the oxygen-doped Be-n (n = 1-12) clusters are obtained at the B3PW91 level. Various energetic and electronic properties of the BenO clusters are systematically investigated using the QCISD(T) method, which are compared with those of pure Ben+1 clusters. The evolution of these properties with cluster size shows the unique stability of Be11O, which can actually be considered as an ionic compound (Be-11)O-2+(2-). On the one hand, O2- has 8 valence electrons, satisfying the octet rule. On the other hand, the Be-11(2+) moiety has a shell-closed electronic configuration, which renders itself particularly stable. (C) 2017 Elsevier B.V. All rights reserved.