A set of 98 nanoporous framework material (NFM) structures was investigated by classical Grand canonical Monte Carlo simulations for low-pressure O2 adsorption properties (Henry’s constant and isosteric heat of adsorption). The set of materials includes those that have shown high O2 uptake experimentally as well as a subset of more than 2000 structures previously screened for noble-gas uptake. While use of the general force field UFF is fruitful for noble-gas adsorption studies, its use is shown to be limited for the case of O2 adsorption—one distinct limitation is a lack of sufficient O2–metal interactions to be able to describe O2 interaction with open metal sites. Nonetheless, those structures without open metal sites that have very small pores (<2.5 Å) show increased O2/N2 selectivity. Additionally, O2/N2 mixture simulations show that in some cases, H2O or N2 can hinder O2 uptake for NFMs with small pores due to competitive adsorption.