A continuous oxygen plasma operating at a radio frequency power of 800 W and a pressure of 2 mTorr has been created in a helicon diffusion reactor used for the deposition of silicon dioxide films. An energy selective mass spectrometer and a Langmuir probe attached to the wall of the silica covered aluminum diffusion chamber below the source have been used td characterize the plasma [ion energy distribution function (IEDF), plasma potential, floating potential, plasma density]. The IEDF of the O-2(+) ions escaping from the plasma to the sidewalls of the chamber consists of a single peak at an energy corresponding to the plasma potential in the chamber (approximate to 32 V). This rather high value is a consequence of the charging effect of the insulating walls at the initiation of the discharge, and would need to be taken account of when estimating the energy of the ions arriving at a biased substrate.