Li-rich (1−x)LiMO2.xLi2MnO3 layered oxides materials have been proposed recently as an attractive alternative to LiCoO2 Their electrochemical performance is greatly improved by charging above the 4.5–4.6 V voltage plateau during the first oxidation. The associated reaction mechanism has been assigned to a transformation from layered to spinel, accompanied a partial oxygen removal process from the surface. Nevertheless, many questions are still unanswered concerning the influence of oxygen extraction and reaction on the chemical composition and evolution of the electrode/electrolyte interphase. Quantitative 7Li, 19F and 31P MAS NMR shows that the partial oxygen extraction at the surface of the electrode material above 4.5 V has a strong influence on the composition/nature of the interphase and exacerbates reactions at the electrode/electrolyte interface. Oxygen reacts as soon as it is extracted from the structure of the positive electrode material to form fluorophosphates in much higher amount in the case of the overlithiated material compared to the stoichiometric material exposed to comparable potentials. The presented study indicates that the release of oxygen from the Li-rich layered oxides greatly influences the electrode/electrolyte interphasial chemistry.