The gas-phase kinetics and plasma chemistry of high density oxygen discharges are studied. A self-consistent spatially averaged model is developed to determine positive ion, negative ion, and electron densities, ground state and metastable free radical densities, and electron temperature as functions of gas pressure, microwave input power, and cylindrical source diameter and length. For an electron cyclotron resonance discharge, the reduction in radial transport due to the confining magnetic field is also modeled. The kinetic scheme includes excitation, dissociation, and ionization of neutrals due to electron impact, electron attachment and detachment, and ion-ion neutralization. In addition, ion neutralization at the reactor walls is included. Model results show that for a low neutral pressure, high plasma density discharge, oxygen molecules are almost completely dissociated to form oxygen atoms, and the dominant positive ion is O+ rather than O2+. The metastable species are not important for the pressure range studied (0.5 to 100 mTorr), and the confining magnetic field significantly affects the plasma chemistry, the total positive ion density, and the electron temperature. Comparisons are made with experimental data, and qualitative agreement between experiment and model is observed.