The adsorption of O-2 and N-2 mixture in nanoporous carbon (NPC) has been investigated by both grand canonical and Gibbs ensemble Monte Carlo simulations. The gases are represented as diatomic molecules, and the NPC is represented as a rigid C-168 Schwarzite structure. Interactions between gas-gas and gas-carbon atoms in the NPC are modeled with additive pairwise site-site Lennard-Jones potentials. Competitive adsorption occurs between the two gases. At fixed bulk composition, the selectivity of O-2 to N-2 first decreases slightly with increasing external pressure, reaches a minimum, and then increases. The isoselective point (ISP) of selectivity reversal is independent of bulk composition. At fixed pressure lower than the ISP, the selectivity monotonically decreases with increasing O-2 bulk composition; however, at fixed pressure higher than the ISP, the reverse is found. With increasing temperature, the selectivity decays approximately exponentially. As with pure gas adsorption, a slight orientational preference of the adsorbed gas molecules toward the intersectional channels is observed, and the center-of-mass density distribution of gas molecules changes from continuous to discontinuous within the NPC as external pressure increases. The predictions of the mixture adsorption using the ideal-adsorbed-solution theory based solely on the adsorption of the pure gases agree well with the simulation results.