Equilibria for the gas-phase clustering reactions of O-2(+), O-2(-), O-4(-), O-3(-), and CO3- with CO were measured with a pulsed electron-beam mass spectrometer. Van't Hoff plots of the equilibrium constants led to the determination of the thermochemical stabilities for cluster ions. QCISD(T)/6-311(+)G(d,p)//UB3LYP/6-311+G(2d,p) calculations were performed to examine cluster geometries and interaction energies. The cluster ion O-2(+)(CO)(n) was found to have the shell structure of O-2(+)(CO)(4)(CO)(4)(CO)(n-8), whereas O-4(-)(CO)(n) has the one of O-4(-)(CO)(4)(CO)(2)(CO)(4)(CO)(n-10). O-3(-) and CO3- ions are solvated preferably by three and two CO ligands, respectively. The nucleophilicity of CO is larger than the electrophilicity. The characteristic nature of the bonding of the cluster ions of CO were compared with that of N-2.