  We analyze the recent experiment [W. Neubeck {\em et al.}, Phys. Rev. B
\vol(60,1999,R9912)] for the resonant x-ray magnetic scattering (RXMS) around
the K edge of Co in the antiferromagnet CoO. We propose a mechanism of the RXMS
to make the $4p$ states couple to the magnetic order: the intraatomic exchange
interaction between the $4p$ and the $3d$ states and the $p$-$d$ mixing to the
$3d$ states of neighboring Co atoms. These couplings induce the orbital moment
in the $4p$ states and make the scattering tensor antisymmetric. Using a
cluster model, we demonstrate that this modification gives rise to a large RXMS
intensity in the dipole process, in good agreement with the experiment. We also
find that the pre-edge peak is generated by the transition to the $3d$ states
in the quadrupole process, with negligible contribution of the dipole process.
We also discuss the azimuthal angle dependence of the intensity.
