p-Nitroaniline (PNA) has been reported as a J aggregate species. In retrospect, this communication confirms a radically different oblique orientation of the PNA units in all three solid, liquid, and gas phases of the dimer, the O-dimer. The nonvanishing transition dipole moments (TDM) associated with the allowed electronic excitations of the O-dimer, computed using electron-hole pair density distribution (EDD and HDD) analyses ascertained the two monomers to be inclined at slippage (π) and polarization (α) angles of 18.5° and 55.4°, respectively. A detailed structure-property relationship of the PNA O-dimeric aggregate was carried out using UV-vis absorption and matrix scan emission spectroscopy, supported by electronic structure calculations at DFT-M062X/6-31G+(d,p) level using integral equation formalism polarizable continuum model (IEFPCM). The computed potential energy surface (PES) implied the global minimum of the PNA O-dimer stabilized by 4.8 kcal.mol-1, owing to bifurcated intermolecular hydrogen bonding. In the excited PNA O-dimeric aggregate, an exchange of excitation energy between the monomeric units resulted in two distinct electronic states separated by an interaction energy of -1644 cm-1. The TD-DFT computed excited state equilibrium structures of the PNA O-dimer corroborated the experimentally observed pronounced Stokes shift to internal conversion following vibrational relaxation of the allowed electronic excited states. On the basis of the detailed structural analysis of PNA O-dimer, the observed energy shifts in optical absorption spectroscopy were evident within the framework of exciton coupling model. © 2015 American Chemical Society.