The properties of titanium oxide thin films deposited by direct current magnetron sputtering of a Ti target are strongly dependent on the sputtering conditions. The aim of the present work is to investigate the discharge parameters such as plasma potential, discharge voltage, deposition rate, and ion composition of the discharge as a function of the oxygen partial pressure. The plasma potential, relative to the ground, is determined from the ion energy distribution. Working in the constant current discharge mode, we observe, with increasing oxygen partial pressure, a drop of the plasma potential, an increase of the discharge voltage, a drop of the deposition rate, and an inversion of the Ti+-TiO+ intensities. For a given discharge current and pressure, the drop of the plasma potential and the increase of the discharge voltage occur at the same gas composition while the drop of the deposition rate and the ion intensity inversion happen at an oxygen richer gas composition. Both transitions are linearly correlated and depend on the discharge current. For larger oxygen partial pressure, a third transition appears corresponding to an inversion between TiO+ and TiO2+ intensities. These results lead us to consider three regimes in the reactive sputtering of a Ti target.