Although cubic rock salt-CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here we present a detailed magnetic and neutron diffraction studies on zinc blende and wurtzite CoO nanoparticles. The zinc blende-CoO phase is antiferromagnetic with a 3rd type structure in a fcc lattice and a Néel temperature of TN (zinc-blende) ~ 225 K. Wurtzite-CoO also presents an antiferromagnetic order, TN (wurtzite) ~ 109 K, although much more complex, with a 2nd type order along the c-axis but an incommensurate order along the y-axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic–like behavior even at room temperature.