Heterometallic metal organic frameworks (MOF) have attracted huge interest for a wide range of applications including gas storage, separation, and catalysis owing to their tunable electronic and magnetic properties. Among several heterometallic MOF structures reported, iron containing MOF structure, namely PCN-250, exhibits excellent thermal and chemical stability. PCN-250 MOF consists of the trimetallic cluster node Fe2M linked with (H(4)ABTC)(6) (H(4)ABTC = 3,3',5,5'-azobenzenetetracarboxylic acid and M = Cr(II), Mn(II), Fe(II), Co(II), Ni(II), or Zn(II)) to form a three-dimensional porous network. In this work, we employed Density Functional Theory (DFT) to investigate the strength of the interaction of O-2 and N-2 gas molecules with both linkers and coordinatively unsaturated metal sites. In addition, grand canonical Monte Carlo simulation is used to predict the adsorption isotherm at two different temperatures, 273 and 298 K, in both homometallic and heterometallic PCN-250. On the basis of the cluster model DFT calculations, we observe almost a factor of 5 selectivity (O-2/N-2) in Fe2Cr- and Fe2Mn-based PCN-250 MOF structures. Incorporation of first-row transition metals with +2 oxidation state showed enhanced binding of O-2 over N-2, correlating well with charge transfer from the metal atom to the adsorbed O-2 molecule. Agreeing qualitatively with DFT calculations, GCMC simulations at 273 K showed higher uptake of O-2 over N-2 following the order Fe2Cr > Fe2Mn > Fe2Ni > Fe2Co > Fe2Zn, respectively. Also, a selectivity of greater than one is predicted for O-2 over N-2 in all heterometallic PCN-250 structures based on a single component adsorption isotherm at 1 bar.