The oxygen solubility in molten silicon was measured in situ by an electrochemical solid ionic sensor over a temperature range from the melting point to 1836 K. The result demonstrates, that the temperature dependency of the solubility cannot be neglected, as might be concluded from previous publications. This is especially important for numerical simulations of the oxygen transport in the melt during the Czochralski silicon crystal growth process, because the oxygen solubility determines the source boundary condition along the crucible wall.