A low-temperature route to pure CoO nanoparticles based on the oxidative decarbonylation of dicobalt octacarbonyl promoted by amine oxides is presented. The small size of the resulting particles (~1.7 nm) leads to spontaneous nanoparticle aggregation. Magnetic measurements show a bimodal behavior with two distinct blocking temperatures, which can be ascribed to the individual CoO nanoparticles and to narrowly size-dispersed aggregates.