Mn films show improved layer-by-layer growth on the Fe(001)-p(1x1)O surface compared to the clean Fe(001) surface. From Auger electron spectroscopy the surfactant role of the oxygen was confirmed. A layerwise antiferromagnetic order in the Mn films is preserved as seen from the magnetic contrast between adjacent Mn layers in spin-polarized scanning tunneling microscopy while the surface spin polarization is enhanced by a factor 2 compared to Mn films grown on the clean Fe(001) surface. Further, topologically induced magnetic frustrations of the Mn layer above buried Fe steps appear wider than without oxygen.