Carbon capture from flue gas emission requires the separation of CO2 from N2 and O2. CO2/O2 separation is more challenging than CO2/N2. O2 permeates faster than N2 because of its smaller kinetic diameter (3.30, 3.46, and 3.64 Å for CO2, O2, and N2, respectively). However, studies on CO2 transport across graphene pores primarily focus on CO2/N2 separations. CO2/O2 separation has received much less attention. For several critical emissions, such as from natural gas combined cycle and cement plant, O2 concentration in the flue gas can be as high as 10%, making this separation important. Therefore, we focus on CO2/O2 separation. CO2/N2 separation was also studied, and results confirmed that CO2/N2 selectivities are much higher than those of CO2/O2.