6SA-CASSCF(10, 10) /6-31G (d, p) and MRCl/cc-pVDZ methods were performed to probe photoisomerization reaction mechanisms of o-nitrophenol. Two low-lying singlet electronic states (S-0 and S-1) and two low-lying triplet electronic states (T-1 and T-2) were found to weave an intersystem crossing network in which a dominant stepwise photoisomerization provides a very efficient reaction pathway; the reaction takes place in the wide region of crossing seam-surface woven by S-1 and T-1 states first, followed by T-1 and S-0 states. Both intersystem crossing regions show strong spin orbital coupling in the order of 40 wavenumbers. All nitro and aci-nitro isomers and transition states on four electronic potential energy surfaces are calculated along with analysis of both dominant and subdominant relaxation pathways, especially weak spin-orbital coupling (similar to 10 wavenumbers) between T-2 and S-1 states and effective conical intersection between T-2 and T-1 states opening a new relaxation pathway S-1 -> T-2 -> T-1.