Adsorption of molecular oxygen with a triplet ground state on Fe -, Co -, Ni -, Ru -, Rh -, Pd -, OS -, Ir - and Pt -doped graphene is studied using density functional theory (DFT) calculations. The calculations show that O 2 molecule is chemisorbed on the doped graphene sheets with large adsorption energies ranging from -0.653 eV to -1.851 eV and the adsorption process is irreversible. Mulliken atomic charge analysis of the structure shows that charge transfer from doped graphene sheets to O 2 molecule. The amounts of transferred charge are between 0.375e- to 0.650e-, indicating a considerable change in the structures conductance. These results imply that the effect of O 2 adsorption on transition metal-doped graphene structures can alter the possibility of using these materials as a toxic-gas (carbon monoxide, hydrogen fluoride, etc.) sensor.