Efficient oxygen activation is crucial for catalytic oxidation reaction, but highly depends on the construction of active sites. Here, dual adjacent Fe atoms anchored on MnO2 can assemble into a diatomic site, which activates molecular oxygen to form an active intermediate species Fe(O = O)Fe for CO oxidation.Efficient molecular oxygen activation is crucial for catalytic oxidation reaction, but highly depends on the construction of active sites. In this study, we demonstrate that dual adjacent Fe atoms anchored on MnO2 can assemble into a diatomic site, also called as MnO2-hosted Fe dimer, which activates molecular oxygen to form an active intermediate species Fe(O = O)Fe for highly efficient CO oxidation. These adjacent single-atom Fe sites exhibit a stronger O-2 activation performance than the conventional surface oxygen vacancy activation sites. This work sheds light on molecular oxygen activation mechanisms of transition metal oxides and provides an efficient pathway to activate molecular oxygen by constructing new active sites through single atom technology.