We have obtained rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra of O-2 in the energy range of 16.0-18.0 eV, covering ionization transitions O-2(+)(a (4)Pi(u), v(+)=0-18,J(+))<-- O-2(X (3)Sigma(g)(.), v "=0,N "). Although these vibrational PFI-PE bands for O-2(+)(a (4)Pi(u), v(+)) have significant overlaps with those for O-2(+)(X (2)Pi(g)) and O-2(+)(A (2)Pi(u)), we have identified all O-2(+)(a (4)Pi(u), v(+)=0-18) bands by simulation of spectra obtained using supersonically cooled O-2 samples with rotational temperatures approximate to 20 and 220 K. While the v(+)=4-18PFI-PE bands represent the first rotationally resolved photoelectron data for O-2(+)(a (4)Pi(u)), the PFI-PE bands for O-2(+)(a (4)Pi(u), v(+)=10-18) are the first rotationally resolved spectroscopic data for these levels. The simulation also allows the determination of accurate ionization energies, vibrational constants, and rotational constants for O-2(+)(a (2)Pi(u), v(+)=0-18). The observed intensities of spin-orbit components for the majority of O-2(+)(a (2)Pi(u), v(+)) vibrational bands are in accordance with the forced spin-orbit/rotational autoionization mechanism.