The ionization energy of the superheavy element Og (Z = 118) and its homolog elements Ar, Kr, Xe, Rn, and their ions are systematically calculated by using the GRASP2K program based on the multi-configuration Dirac-Hartree-Fock (MCDHF) method, taking into account relativistic effects, electron correlation effects between valence shell electrons, quantum electrodynamics effects, and Breit interaction. To reduce the uncertainty of the ionization energy derived from electron correlation effects which are not fully considered, the ionization potential of the superheavy element Og^0–2+ and its homolog element Rn^0–2+ are extrapolated by the extrapolation method. The ionization energy of extrapolated Rn^0–5+ and Og⁵⁺ coincide well with experimental and other theoretical values. These results can be used to predict the unknown physical and chemical properties of the atoms and compounds of the superheavy element Og. In addition, the calculation results of the electron orbital binding energy of the atomic valence shell of the superheavy element Og and its homolog elements Ar, Kr, Xe, and Rn under relativistic and non-relativistic conditions show that owing to the relativistic effect, there occur strong orbital contraction phenomena in the 7s orbital and 7p_1/2 orbital and strong splitting phenomena in the 7p_1/2 orbital and 7p_3/2 orbital of Og, which may cause the physical and chemical properties of the superheavy element Og to differ from those of other homologs.