The magnetism in solid oxygen, especially in different high-pressure phases, is not completely understood but can be studied indirectly by spectroscopic techniques. Therefore, we measured and modeled pressure effects on the optical band shape of vibronic two-molecule transitions (3)Sigma(g)(-3)Sigma(g)(-) --> (1)Delta(g)(3)Sigma(g)(-). Corresponding spectra at ambient and elevated (P < 10 GPa) hydrostatic pressure and low temperature were very complex but could be clearly assigned to a zero phonon line (ZPL), which in solid oxygen is presented by the exciton-magnon doublet, and its phonon sideband. After the deconvolution of the spectra (ZPL and sideband), the frequency and integrated intensity of the band origin were plotted as functions of pressure. We tried to develop a theory and to model the combined excitation (exciton + magnon + phonons); then we could derive theoretical relations that we compared to experimental data: (i) the splitting of the exciton-magnon doublet increases with pressure because of the growth of all (resonance and exchange) interactions in the basal ab-plane; (ii) the integrated intensity of the ZPL decreases with pressure because the equilibrium positions of molecular oscillators in the ground and optically excited crystal states are shifted relative to each other; (iii) the total band intensity increases with pressure as the combined excitations are assisted by intermolecular exchange interaction due to the decrease in the distance between molecules participating in this transition.