The threshold photoelectron spectrum of oxygen has been measured with high resolution in the photon energy range of 12-19 eV using a penetrating field technique and a synchrotron radiation source. The long vibrational progression of the X-2 Pi(g) state of O-2(+) was observed to upsilon'=45. An extrapolation in a Birge-Sponer plot (Delta G versus upsilon'+1/2) gives a maximum vibrational level upsilon'(max)=56. The A(2) Pi(u) state was also measured in detail. Most vibrational levels of the A(2) Pi(u) state of O-2(+) (from upsilon'=0 to 24) were observed for the first time. A Birge-Sponer plot of the A(2) Pi(u) state gives a maximum vibrational level upsilon(max)'=31. The dissociation limit of the A(2) Pi(u) state is slightly higher (76 meV) than the well-known dissociation limit O(P-3(2))divided by O+(S-3(0)) 18.733 eV. Therefore the A(2) Pi(u) state may be considered to have a hump. We observed new states which were assigned to (4) Sigma(g)(+) and (2) Sigma(u)(+) according to a previous theoretical investigation [Beebe et al., J. Chem. Phys. 64, 2080 (1976)]. Potential curves of the X-2 Pi(g) and A(2) Pi(u) states were calculated by the Rydberg-Klein-Rees method using both the rotational constants obtained previously by others [K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, Constants of Diatomic Molecules Volume TV (Van Nostrand, New York, 1974)] and the vibrational energy values observed in the present experiment. A Le Roy-Bernstein plot as well as a Birge-Sponer plot were used to determine the dissociation energy of the X-2 Pi(g) state. However, the plot gave an unreasonably large dissociation limit. (C) 1997 American Institute of Physics.