[en] Combining experiments and first-principle calculations, we present in this paper a detailed study of the O_3 detection mechanism on the CuO (111) surface. The exchange-correlation functional is treated within both the LDA and the GGA including the spin polarization. In order to better take into account the on-site electronic interactions between 3d electrons of Cu atoms a Hubbard term U has to be added in all calculations. We show that the O_3 molecule is reduced to an O_2 molecule with an enthalpy of reaction of -1.11 eV (-1.15 eV) within LDA + U (GGA + U). Along the reaction path, the O_3 molecules are first physi-sorbed with a large adsorption energy of -1.83 eV (-1.03 eV) and a significant charge transfer from the surface to the molecule. The p-doping strengthening is compared to the electrical response of a CuO based sensor under O_3 exposure. (authors)