Magnetic Arc Oscillation was applied during the construction of single-pass multi-layer walls of low carbon steel and Ti6Al4V by the Gas Tungsten Arc Welding-based Wire and Arc Additive Manufacturing process, and the influence on the geometry and the process stability was evaluated. The geometric features were assessed using transverse section macrographs and the effects of different patterns and frequencies of oscillation on the arc characteristics, metal transfer and weld pool behavior during the layer deposition were investigated using high speed and welding cameras. The results show that this technique is efficient for making thinner walls by minimizing the overflow of the weld pool and, in some conditions, increasing deposition efficiency. Furthermore, the distribution of material along the wall length becomes more homogeneous. An explanation of the effects of Magnetic Arc Oscillation on the wall geometry based on forces that act on the molten metal during layer deposition was made. Because of the swinging movement of the welding arc, the heat is distributed over a larger area, and the power density decreases. Thus, fewer previous layers are remelted, and the volume and the weight of the weld pool reduce. The weld pool temperature drops, and the surface tension force and the viscous friction increase. The distribution of arc pressure also becomes less concentrated, and the arc force on the molten metal decreases. Additionally, a magnetic force appears on the molten metal, which contributes to a change in the direction of the resultant force on the weld pool.