The adhesion of atmospheric silicon (Si) to the highly polar gallium nitride (GaN) regrowth surface leads to the formation of a parasitic, conductive channel in lateral GaN-on-GaN devices. This parasitic channel can result in high transistor off-state leakage currents, impeding the use of GaN as substrate material for certain applications. Ex-situ cleaning methods to remove the surficial Si effectively were unsuccessful, because of fast re-adsorption from the ambient atmosphere. Therefore, establishing a passivation for GaN after an in-situ cleaning step, which can be desorbed prior to GaN/AlGaN heterostructure growth, is considered to be a technical solution. The formation of a thin passivating gallium oxide layer on the GaN surface is demonstrated by oxygen plasma oxidation. While an amorphous character of the passivation layer is needed for subsequent desorption, the oxygen plasma treatment resulted in a stable crystalline phase preventing desorption. Moreover, the plasma treatment causes significant damage to the GaN surface and introduces additional impurities. Specifically, these two aspects – the introduction of additional impurities and defects at the GaN surface and the crystalline character of the oxide – prevented removal of the Si contamination at the GaN/GaN regrowth interface. Nevertheless, the experiments provide valuable insights into one possible oxidation mechanism and particularly point at the necessity of establishing an amorphous gallium oxide passivation layer, which will be the focus of future work.