Boron-centered radicals have received growing interest. Recently, two groups reported density functional theory investigations (GGA-PW91 and B3LYP) on a hexa-atomic boron-oxide radical, B 5 O, which has led to great discrepancies on the type of low-lying structures. In this article, we not only explore the energetics of doublet and quartet B 5 O isomers at high electron-correlated levels (CCSD(T)/6-3111G(2df), CCSD(T)/aug-cc-pVTZ, and G3B3) but also investigate the isomerization and fragmentation stability of the low-lying B 5 O isomers. All the high-level studies consistently show that the B 5 O radical possesses a belt-like ground structure 2 01 in doublet electronic state followed by isomer 2 02 with an exocyclic Γ ΓBO moiety at around 3.0 kcal/mol. Kinetically, 2 01 and 2 02 are separated by a considerable barrier of about 20 kcal/mol. Thus, the two isomeric forms of B 5 O radical should be very promising for isolation in laboratory. However, the other four isomers reported recently are all kinetically unstable toward conversion to 2 01 and 2 02. The high thermodynamic and kinetic stability of 2 01 and 2 02 might make them as important building cores in the growth of boron-oxide clusters. This results would also help deeply understand the oxidation and doping mechanism of pure boron clusters.