We present a theoretical and experimental analysis of non Franck-Condon effects in photoionization to the b 4Σ-g state of O+2. Experimentally, by dispersing the synchrotron radiation induced O+2 b 4Σ-g - a 4Πu fluorescence we derive the b 4Σ-g state vibrational branching ratios in the excitation energy range 21-34 eV. The vibrational branching ratios reveal features in the region 21-28 eV, indicating strong non Franck-Condon effects. The experimental results have been analysed by computing ab initio the vibrational population branching ratios using a many-body perturbation method. Additionally, the autoionizing neutral states existing in this energy region have been studied. We have computed the energies of the valence states up to an energy of 30 eV, their transition moments for excitations from the ground state, and autoionization rates. Our calculations show, that strong non Franck-Condon effects recorded in the branching ratio spectrum (below 25 eV) are actually caused by the narrow 3σgσu shape resonance, and its coupling to the 1πuπg channel.