In this work, the O-2/N-2 adsorption behavior of ultramicroporous MOF Cu(Qc)(2), with preferential adsorption of O-2 over N-2, was investigated for the first time. Cu(Qc)(2) was prepared using a room-temperature synthesis strategy and then characterized. High-pressure isotherms of O-2 and N-2 were measured at different temperatures. The molecular simulation was used to reveal the adsorption mechanism of O-2 and N-2 on the copper sites of Cu(Qc)(2). Inverse gas chromatography (IGC) experiments were applied to measure surface free energy for O-2 and N-2 adsorption for evaluation of the affinity between adsorbates O-2/N-2 and Cu(Qc)(2). Results showed that the adsorbed amounts of O-2 on Cu(Qc)(2) increased significantly with pressure rising, reaching as high as 4.48 mmol/g at 259 K and SO bar, while that of N-2 increased slowly to 0.98 mmol/g. The uptake ratio of O-2/N-2 on Cu(Qc)(2) reached 4.62 at 298 K and 50 bar. The LAST-predicted O-2/N-2 (21:78 v/v) selectivity of Cu(Qc)(2) reached 7 at 259 K and 50 bar. The ultramicroporous MOF Cu(Qc)(2) is the potential for separation of O-2/N-2 from the air.