Monochromatic UV light in the spectral interval between 4.0 and 5.5 eV is used in order to selectively excite 3- and 4-coordinated oxygen anion sites on the surface of MgO nanoparticles exposed to O-2 gas. As a result, two different paramagnetic O- surface species and also ozonide anions O-3(-) are observed by electron paramagnetic resonance (EPR) spectroscopy. The relative abundance of each of the O- species exhibits a specific dependence on the energy of the exciting photons. EPR data together with the results of theoretical modeling suggest that both O- species are located at 3-coordinated sites having different local environments. At sufficiently high O-2 pressures molecular oxygen does not only act as an electron trap, favoring the O- formation, but it also contributes to UV induced O-3(-) formation with a maximum efficiency at 4.2 eV.