For full oxygen monolayer coverage on Fe(100), the interlayer distance between the first and second iron layers increases from 1.39 Å to 1.66 Å (experimental: 1.54 +/-0.06 Å). Moreover, it was further demonstrated the enhancement of magnetic moments of the top surface layer upon the adsorption of oxygen. The increment in the magnetic moment has reached the magnitude of those in the antiferromagnetic iron oxides.