A theoretical analysis of the behaviour of molten metals in the presence of oxygen is presented. A generalised Wagner approach has been adopted for molten metals, forming volatile oxides (Si, Sn, Al, etc.), in which the description of oxygen transfer from the gas phase to the condensed phase must account for the double contribution of molecular oxygen and oxygen linked as oxide, which leads to define the concert of 'oxygen effective pressure'. From the analysis of the system at varying operating conditions, it was possible to relate the gas-phase composition at the surface, which is hardly measurable, to the composition in the feed or at the outlet. The application of this theory to the molten silicon-oxygen system is presented and the effectiveness of such a theory as a supporting tool for experimental work of capillary phenomena and crystal growth processes is discussed and verified.