The np (n=3-10) and nf (n=4-9) Rydberg states of O-2 converging on O-2+ X (2)Pi(1/2,g) and X (2)Pi(3/2g) have been studied between 75 000 and 99 900 cm-1 using optical-optical double resonance with multiphoton ionization. Three-photon excitation from single rotational levels of the initially excited b (1)Sigma(0g)(+) valence state was used to access these states. The nf states show a strong tendency towards (Omega,omega) coupling for all values of n, whereas the np states appear to be best described by (Lambda,S) coupling for nless than or equal to8. The intensities of some of the 5f bands are anomalously high due to accidental resonances with the 3s d (1)Pi(1g) Rydberg state at the two-photon level. (C) 2003 American Institute of Physics.