We present an ab initio study of structural, electronic, and magnetic properties of rocksalt cobalt oxide doped with 3d transition metal atoms (Mn, Fe, and Ni). Our calculations are based on the generalized gradient approximation to the density-functional theory corrected for on-site Coulomb interaction (GGA+U). Different ab initio approaches based on the plane-wave basis set are applied. Structural and magnetic characteristics of pure CoO are calculated and compared with available experimental and theoretical results. Magnetic states of single impurities in CoO are revealed. The interaction between impurities is discussed. Our study demonstrates that both single impurities and their pairs do not break the type-II antiferromagnetic order of the host CoO.