On the basis of rotationally resolved pulsed field ionization photoelectron measurements, we have shown that the weak vibrational progressions in the energy region of 19.6-21.0 eV observed in recent high resolution photoelectron spectroscopic studies of O-2 are associated with transitions to the 2 (2) Pi(u 1/2) (2 (2) Pi(u 3/2)) and 2 Sigma(u)(-)states of O-2(+), with ionization energies of 19.6268+/-0.0005 eV (19.6329+/-0.0005 eV) and 20.3528+/-0.0005 eV, respectively.