The characteristics of a low frequency (70 kHz) discharge with non-symmetrical configuration of electrodes in He–O2 mixtures were investigated. The Langmuir probe technique, emission spectroscopy, mass spectrometry and absorption spectroscopy were used for plasma diagnostics. The physicochemical properties of PET (polyethylene terephtalate) were analyzed via contact angle measurements and XPS spectroscopy. The presence of the oxygen is shown to modify both the densities of the helium metastable states and also the energetic character of the discharge. The very efficient loss of He excited species and in particularly the metastables, for amounts of O2 higher than a few percents, affects the surface modification in terms of crosslinking, and therefore the stability. This work has shown that one of the important species responsible for the crosslinking process leading to stable surface properties could be the helium metastables.