Laser-ablated Co atoms react with Oz to give the linear dioxide, the monoxide, and the rhombic (CoO)(2) dimer molecules. The linear OCoO molecule is characterized by oxygen-18 substitution in strong nu(3) (945.4 cm-(1)) fundamental and weak nu(1)+nu(3) (1717.6 cm(-1)) combination bands. Annealing to 20-30 K to allow diffusion and reaction of cold cobalt atoms and dioxygen markedly increases weak bands due to the Co(Oz) and CoOO complexes and higher species including OOCoO, OOCoO2, (O-2)CoO2, and the dimer (CoO2)(2) DFT and CASSCF calculations performed on the three CoO2 isomers and rhombic (CoO)(2) support these assignments; DFT calculations are also presented for the rhombic (FeO)(2) molecule.