The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to 28,000wn and for temperatures 8 $< T <$ 325 K@. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at $T_N$ = 289 K has been observed. At low temperatures the splitting amounts to 15.0wn. In addition, we studied the splitting of the cubic crystal field ground state of the Co$^{2+}$ ions due to spin-orbit coupling, a tetragonal crystal field, and exchange interaction. Below $T_N$, magnetic dipole transitions between the exchange-split levels are identified and the energy-level scheme can be well described with a spin-orbit coupling $lambda = 151.1wn$, an exchange constant $J = 17.5wn$, and a tetragonal crystal-field parameter $D = -47.8wn$. Already in the paramagnetic state electric quadrupole transitions between the spin-orbit split level have been observed. At high frequencies, two electronic levels of the crystal-field-split $d$-manifold were identified at 8,000 and 18,500wn.
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