A spin-polarized metastable deexcitation spectroscopy apparatus using a nozzle-skimmer pulsed discharge metastable atom source was developed. The oxygen adsorption dependence of the surface magnetism of thin iron films deposited on MgO(100) was investigated using this apparatus. The surface local density of states spilling towards the vacuum (SDOS) at around the Fermi energy, EF, for the clean surface shows a negative polarization to the bulk. The oxygen derived SDOS for the Lightly oxygen adsorbed surfaces (2-10 L) also shows a negative polarization while the SDOS at EF changes its polarity to positive. The polarization of SDOS is not detected for the heavily oxidized surfaces (20-100 L).