Ternary boron-carbon-nitride compounds are the hardest, chemically stable, and most applicable semiconductors in optoelectronic devices. We investigate the quasi-particle and excitonic properties of type II o-BCsub.2N using many-body perturbation theory (MBPT). The state-of-the-art GW and BSE methods were used to determine the accurate band gap and excited-state characteristics of this material. We simulate the convergence test and structural optimization in DFT, which is the starting point for the GW calculation. We also compute the convergence test of the parameters in GW and BSE. As a result, the bandgap of our system is found to be 2.31eV and 1.95eV using the GW approximation and DFT-PBE, respectively. Since the valence and conduction band edges are located at different Brillouin zones, we decide that o-BCsub.2N is an indirect bandgap semiconductor. In addition, by applying the scissor operator, we corrected the quasi-particle bandgap, which shows almost the same result as the GW approximation. Furthermore, using the BSE algorithm, we calculate the optical bandgap of type II o-BCsub.2N to be 4.0eV with the excitonic effect and 4.4eV without the excitonic effect. The highest peaks of the imaginary dielectric function with the excitonic effect shift to a lower energy level at 11eV than without the excitonic effect at 13.5eV. The electron charge distribution is computed by fixing the hole position. Finally, we suggest that type II o-BCsub.2N is promising for the application of optoelectronic semiconductors.