Nonmass-dependent isotopic enrichment of 17O and 18O has been observed mass spectrometrically in O4+ ions produced by termolecular association reactions of O2+ and O2, where the O2+ was generated by electron ionization. The enhancement is strongly dependent on the energy of the ionizing electrons, decreasing from a near tenfold enhancement at threshold for O 2+ production to no enhancement above 40 eV. Additionally, O2+ generated near threshold were found to be significantly less efficient in producing O4+ than O 2+ ions generated at higher energies. A permutation-inversion, symmetry analysis of the termolecular association reaction suggests that the results can be understood in terms of a symmetry restriction on the O2+(2Πg) rotational states which can efficiently access the electronic ground state of O4+ upon collisions with O2. The restriction is rooted in the Pauli principle and vanishes when the O2+ ion is isotopically heteronuclear. The possible relevance of the nonmass-dependent isotope enhancement in O4+ to similar enhancements found in stratospheric and laboratory-produced ozone is discussed.