The interplay between aliovalent CuO doping and nonstoichiometry on the development of defect structures and the formation of secondary phases of antiferroelectric NaNbO3 ceramics has been investigated by means of x-ray diffraction (XRD), first-principles calculations using density functional theory (DFT), and electron paramagnetic resonance spectroscopy. The results indicate that, for stoichiometric 0.25 mol% CuO-doped NaNbO3, as well as for 2.0 mol% Nb-excess sodium niobate, the Cu2+ functional centers are incorporated at the Nb site (Cu'''(Nb)). For reasons of charge compensation, two kinds of mutually compensating defect complexes (Cu'''(Nb) - V-O(center dot center dot))' and (V-O(center dot center dot) - Cu-Nb(center dot center dot) - V-O(center dot center dot))(center dot) are formed where, for the niobium-excess compound, additionally, V'(Na) contribute to the mechanism of charge compensation. In contrast, for 2.0 mol% Na-excess sodium niobate, a Na3NbO4 secondary phase has been detected by XRD, and only part of the Cu2+ forms these types of defect complexes. The major part of the Cu2+ is incorporated in a fundamentally different way by forming Cu2+-Cu2+ dimeric defect complexes.