The electron-spin magnetic moments of O-3(-), O3Li, and O3Na, as parametrized by the g factors, are studied at the uncorrelated restricted open Hartree-Fock (ROHF) and correlated multireference configuration interaction (MRCI) ab initio levels. The present method, which uses a perturbative approach complete to second order, is based on a Breit-Pauli Hamiltonian. The calculated Delta g values, with Delta g=g-g(e), are very similar for all three species, confirming that the O-3(-) moiety is retained in the ozonides O3M. In the standard C-2v notation, Delta g(yy)>Delta g(zz)much greater than\Delta g(xx)\. The perpendicular component Delta g(xx) is small and negative, while the in-plane components Delta g(yy) and Delta g(zz) are large and positive. The MRCI results for Delta g(xx), Delta g(yy), Delta g(zz) (in ppm) are : -475, 16 673, 10 121 for O-3(-); -679, 13 894, 9308 for O3Li; and -494, 12 298, 8690 for O3Na. The ROHF values of Delta g(yy) and Delta g(zz) are smaller than the MRCI data, due to a general overestimation of the excitation energies. The MRCI Delta g values for isolated O-3(-) reproduce the experimental results for O-3(-) trapped in crystals or adsorbed on MgO surfaces (in ppm, -500-1200 for Delta g(xx), 12 400-16 400 for Delta g(yy), and 6900-10 000 for Delta g(zz)). For O3Na, the experimental Delta g(xx), Delta g(yy) Delta g(zz) data (-100, 14 200, 9800 ppm) are again satisfactorily described by our correlated results. No experimental g shifts are available for O3Li. In all systems studied here, the Delta g(xx) component is dictated by first-order terms (ground state expectation values); Delta g(yy) is governed by the second-order magnetic coupling between X B-2(1) and 1 (2)A(1) [electron excitation from the highest a(1) molecular orbital (MO) into the b(1)(pi*) singly occupied MO]; and Delta g(zz), by the coupling with two B-2(2) states (excitations from the two highest b(2) MOs into pi*).