We have obtained high-resolution photoelectron spectra for eight of the ten 3d pi electronic states of O-2, using a rotationally resolved, two-color 2+1’ excitation and ionization scheme. A magnetic bottle photoelectron spectrometer, used to acquire the spectra, provided a resolution sufficient to observe the yields of the individual spin-orbit states of the ground electronic state of the ion. Branching ratios were obtained from the spectra for three different photoionization wavelengths. The overall behavior of the branching ratios for different electronic states can be explained by calculating the core state composition of the intermediate states, taking into account the spin-orbit mixing of Hund’s case (a) basis states, and assuming that electronic states having the core state Omega(c)=3/2 have a larger photoionization probability than those having an Omega(c)=1/2 core.