The stability of isolated metal atom on a support plays very crucial role to avoid ensemble effects of agglomeration and clustering. Defects in support matrix provide additional binding to enhance the stability of single atom catalyst. Here, using plane wave based pseudo-potential approach, we report the activation of oxygen molecule by Co, Ni and Cu embedded h-BN sheet with both B and N vacancy. We have used O-O bond stretching as probe for the activation. The results show that the stability of these metal atoms is higher at the vacancy site than on pristine h-BN. The atomic charge distribution of the metal embedded h-BN sheet suggests that while for boron vacancy defects metal atoms donate electronic charge, for nitrogen vacancy defects metal atoms accept electronic charge from the sheet. The chemical reactivity of the M doped h-BN sheet has been analyzed through electronic density of states spectrum. The interaction of O2 molecule with metal supported h-BN sheet suggests that the interaction energy as well as the O-O bond stretching is higher on metal atom at the nitrogen defected site. A case study of CO oxidation on the Cu embedded NV sheet has been demonstrated by co-adsorption of CO and O2 molecules.