Oxygen is used as a surfactant to improve the layer-by-layer growth of ultrathin ferromagnetic Fe, Co, and Ni films. During this process 0.5 monolayers of oxygen ions are floating on top of the ferromagnetic film. XMCD enables us to study the magnetism of such films at their L2,3 edges and in the same experiment the induced magnetic moment of the oxygen at the K edge. In addition, NEXAFS at the O K edge is used to investigate the bonding of the adsorbate to the 3d metal. The experimental findings are combined with DFT calculations to reveal the crystallographic and electronic structure as well as the magnetic properties of Fe, Co, and Ni monolayers grown with oxygen surfactant.