Adsorption of molecular oxygen on CuN (N = 2-10) clusters is investigated using density functional theory under the generalized gradient approximation of Perdew-Burke-Ernzerhof. An extensive structure search is performed to identify low-energy conformations of CuNO2 complexes. Optimal adsorption sites are assigned for low-energy isomers of the clusters. Among these are some new arrangements unidentified heretofore. Distinct size dependences are noted for the ground state CuNO 2 complexes in stability, adsorption energy, Cu-O2 bond strength, and other characteristic quantities. CuNO2 with odd-N tend to have larger adsorption energies than their even-N neighbors, with the exception of Cu6O2, which has a relatively large adsorption energy resulting from the adsorption-induced 2D-to-3D structural transition in Cu6. The energetically preferred spin-multiplicity of all the odd-N CuNO2 complexes is doublet; it is triplet for N = 2 and 4 and singlet for N = 6, 8, and 10.