The technique of dual control volume grand canonical molecular dynamics (DCV GCMD) is used to study the diffusivity of a mixture of nitrogen and oxygen in a graphite slit pore as a function of pore width. We find evidence in support of combined viscous and diffusive transport through these narrow slit pores. The viscous contribution to the flow becomes weaker as the pore width decreases. The fluid velocity profiles show evidence of microscopic slip but still retain a classical Navier-Stokes parabolic signature. The concentration profiles for each component in the mixture show an approximately linear variation with distance along the pore length, suggesting that cross-coupling effects are weak. We find that the diffusion of oxygen -nitrogen mixtures through a graphite pore displays a complex dependence upon the pare width. Molecular packing appears to(-) play a very significant role in determining the flow of the mixture. Thermodynamic effects are of more importance in these simulations than sieving effects; hence, we do not find a greater diffusivity for oxygen compared to nitrogen.