Highly accurate single-crystal structure factors, complete up to sin θ/λ=1.6 Å-1, have been measured from CoO at 305 K and in the antiferromagnetic state at 10 K using 316.5 keV γ radiation. Moderate uniaxial pressure was applied in the low-temperature phase to force a single-T-domain sample. A detailed description of the electron-density distribution is presented. The occupancies of the 3d shell are computed from multipole refinement parameters, showing significant differences between the two magnetic phases. The spin and orbital contributions to the total magnetic moment of Co are derived from the experimentally determined number of unpaired electrons. Results are compared with ab initio calculations as well as with magnetic form factor measurements. A careful analysis of electronic properties in the internuclear regions reveals the Co-O interaction to be purely ionic. Its physical significance on the antiferromagnetism is discussed.