A systematic study of a low pressure (5--200 mTorr) oxygen discharge is presented. Measurements of the electron and negative ion densities in the 13.56 MHz capacitively coupled plasma are performed by means of a microwave resonance technique in combination with photodetachment. A kinetic model is developed, incorporating volume and wall reactions of ions as well as neutral species. It is shown, by matching the experimental results with the model, that the dominant ion is ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ whereas the ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{O}}_{3}^{\mathrm{\ensuremath{-}}}$ densities reach 10 to 20 % of the total negative ion density. The ratio of negative ion to electron density varies between 5 and 10 and decreases with pressure and rf power. The total negative ion density is about 5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$, it increases with gas flow, is independent of the rf input power, and has a maximum at a pressure of 30 mTorr. The agreement between the measurements and the model is within the experimental error for a wide range of conditions. Deviations are explained by changes in the gas and wall temperatures. From the known ion density the effective ionization rate has been determined and related to the electron temperature. The ionization temperature obtained in this way varies between 2 and 4 eV and decreases with increasing gas pressure and power, as expected for the bulk electron temperature in this type of discharge.