MOFs can separate O-2/N-2 mixtures via various mechanisms, making them a great candidate for many air separation applications. We show that the adsorptive behavior of MOFs can be classified based on whether they contain open metal sites (OMSs) and if those sites form a coordination bond with oxygen molecules. Existing data suggests that OMS MOFs that can bind O-2 have high O-2/N-2 selectivity, while OMS MOFs that cannot form such a bond have increased selectivity toward N-2. Monte Carlo simulations show that saturated metal site (SMS) MOFs selectively adsorb O-2 based on dispersion forces. Inaccuracies in simulations on nonbinding OMS MOFs are suggested as evidence that there are other nonbinding interactions impacting O-2/N-2 adsorption in OMS MOFs. Monte Carlo simulations on SMS MOFs are used to define a volume-weighted average pore diameter to establish clear correlations between complex pore size distributions and adsorptive properties.