Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed to gain insight into the structure of poly(o-phenylenediamine) (POPD). Both reported structures of POPD, ladder (L)- and polyaniline (P)-like, are investigated theoretically through the oligomers approach. The simulated vibrational properties of 5POPD(L) and 5POPD(P) at B3LYP/6-31G (d) along with their assignments are correlated with experimental frequencies. Vibrational spectra show characteristic peaks for both POPD(L) and POPD(P) structures and do not provide any conclusive evidence. Excited-state properties such as band gap, ionization potential, electron affinities, and HOMO–LUMO gaps of POPD(L) and POPD(P) from monomers to five repeating units are simulated. UV–vis spectra are simulated at the TD-B3LYP/6-31+G (d, p) level of theory, supportive to the ladder-like structure as the major contributor. Comparison of the calculated data with the experimental one strongly suggests that the ladder-like structure is the predominant contributor to the molecular structure of POPD; however, a small amount of POPD(P) is also believed to be present.