Photoelectron spectroscopy of size-selected ConOm- clusters (n = 4-20; m = 0-2) is performed in a pulsed molecular beam experiment at photon energies of 3.49 and 4.66 eV. Cobalt oxide cluster anions are generated in a laser vaporization source and the photoelectron spectra are measured with a magnetic bottle photoelectron spectrometer. The size-dependent evolution of the electronic structure of the oxide clusters is discussed in terms of cluster composition. Threshold binding energy features are mainly contributed from Co d-defived orbitals throughout the complete series, indicating that the incoming oxygen atoms cause a minor influence on the electronic structures. Although for n greater than or equal to 13 EAs and VDEs of Co-n and ConOm clusters overlap within the experimental resolution, at smaller cluster sizes, the values increase in the order of Co-n < ConO < ConO2. For Co4O3- and Co5O3-, the spectra indicate the presence of isomers with low EAs as peroxo and ozonide isomers. To compare the different channels of oxygen chemisorption, relative dissociation energies are determined via thermochemical cycles.