Homoepitaxial magnesium oxide thin films have been grown on MgO(100), (110) and (111) substrates by the plasma enhanced metal-organic chemical vapor deposition. Atomic force microscopy measurements indicate that the as-received MgO substrate surface morphology is greatly impro\-ed as a result of MgO homoepitaxial growth. Various oxygen partial pressures and plasma powers were selected to investigate the role of atomic oxygen species during the growth of the MgO thin films. Oxygen plasma conditions were analyzed by Langmuir probe measurements. These studies reveal that higher concentrations of atomic oxygen effectively assist the layer-by-layer growth kinetics, wt-rich are critical to generating atomically flat surfaces. Smooth surfaces, with roughness as low as 0.036 nm, are achieved on MgO(100) substrates under the conditions of 2.1 mTorr oxygen partial pressure and 1300 W plasma power at 720 degreesC.