Mixed Mn–Co oxide nanoparticles have been prepared by decomposition of metal carboxylate precursors in a mixture of oleic acid (OA) and tri-n-octylamine (TOA). In addition to the stable MO rock salt structures, wurtzite-type Co1−xMnxO (0.55 < x < 0.65) nanoparticles have been synthesized for the first. Molar ratios of OA to precursor in excess of 5 were required to obtain the metastable phase. Varying the surfactant, water and precursor ratios allows control of particle size, morphology and stoichiometry: diamond cuboid, rhombohedra and hexagonal-shaped nanocrystals as well as branched rods of rock salt Co1−xMnxO (0.01 ≤ x ≤ 0.9) have also been synthesized. The rhombohedra and hexagons exhibited dimensions <50 nm on the longest axis, while nanorods had branches up to 150 nm and cuboids grew up to 250 nm on an edge. These results indicate that nanoparticle morphologies and crystal lattices arise from crystal growth and Ostwald ripening at different rates selecting for either small, smooth-surfaced wurtzite nanoparticles or large, dendritic rock salt nanoparticles. In some mixed metal Mn1−xCoxO particles, superlattice ordering is observed. As with the previously studied MnO and FeO systems, surface oxidation occurs to produce spinel phase M3O4.