The ion mass spectrometer on Cassini detected enhanced ion flux near Saturn's main rings that is consistent with the presence of atomic and molecular oxygen ions in the thermal plasma. The ring atmosphere and ionosphere are likely produced by UV photosputtering of the icy rings and subsequent photoionization of O2. The identification of the O+ and O2+ ions is made using time-of-flight analysis and densities and temperatures are derived from the ion counting data. The ion temperatures over the main rings are a minimum near synchronous orbit and increase with radial distance from Saturn as expected from ion pick up in Saturn's magnetic field. The O2+ temperatures provide an estimate of the neutral O2 temperature over the main rings. The ion mass spectrometer also detected significant O2+ outside of the main rings, near the F ring. It is concluded that between the F and G rings, the heavy ion population most likely consists of an admixture of O2+ and water group ions O+, OH+, and H2O+.