With electron propagator methods, electronic ionization and attachment energies are obtained directly. The multiconfigurational spin tensor electron propagator method (MCSTEP) is explicitly designed for systems with open shell and/or nondynamical correlation in the initial state. We apply MCSTEP to O-2 at several internuclear separations and obtain and report the MCSTEP potential energy curves and the spectroscopic constants for the X(2)II(g),a(4)II(u),A(2)II(u),b(4) Sigma(g)(-),B-2 Sigma(g)(-),(IIu)-I-2, and c(4) Sigma(u)(-) states of O-2(+).