The present work aimed at evaluating the effect of O2 content in argon-based shielding gases over the arc cathodic emission behavior (which can lead to arc wandering) in wire + arc additive manufacturing (WAAM) of thin aluminum walls and its consequences over layer formation. The effect of O2 content on arc wandering was assessed by analyzing cathodic spot behavior through high-speed videography during Gas metal arc (GMA-AM) depositions. Superficial and geometric aspects were analyzed, as well as the sputtering zone width on the wall sides. Through a proposed model, a parallel effect of wire-introduced oxides was weighted to explain arc wandering, searching for oxides in the wall sides (deviating from its action only on the pool). The main finding was that no influence of O2 from the shielding gas was observed on arc wandering, consequently on layer formation, when its content was up to 200 ppm. When an O2 content of 20,000 ppm was employed, oxide searching by the arc in the wall sides was no longer perceived due to enough oxide availability from the shielding gas. However, this favorable condition to layer formation entailed excessive layer oxidation. From the cathodic emission point of view, it can be said that high-purity shielding gases or special mixtures with small additions of O2 (up to 200 ppm) provide no significant advantages for WAAM of aluminum thin walls, at least for the hereby tested alloy and parameters.