The factor f(w)(T,p) by which oxygen increases the saturation vapor pressure of water has been measured accurately for temperatures of 25, 50, and 75 degrees C and pressures to 14 MPa. This paper outlines the measurements and describes their analysis to give the interaction second virial coefficient B-OOW(T) and the interaction third virial coefficient C-OOW(T) of the vapor-gas mixture. Because the work largely parallels the authors’ work for water in air, already reported in detail, the present paper is relatively brief. Fitting Lennard-Jones potentials to the values of B-OW(T) for the three temperatures shows the values to be highly mutually consistent and allows this coefficient and its derivatives to be calculated over a wide temperature range. The three values of C-OOW(T) define the magnitude of this quantity well for temperatures from about 0 to 100 degrees C, but for lack. of a reliable theory of the form of its temperature dependence yield little information concerning its derivatives. No earlier published measurements or theoretical values of the coefficients appear to exist. The work allows the vapor-pressure enhancement factor f(W)(T,p) to be calculated for a wide range of conditions and provides a basis for the calculation of the thermodynamic properties of the water-oxygen system.