Laser-ablated Co atoms react with O2 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 ν3 (945.4 cm-1) fundamental and weak ν1+ν3 (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(O2) and CoOO complexes and higher species including OOCoO, OOCoO2, (O2)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.