To quantify the oxygen content in molten salts, we examined the performance of an yttria-stabilized zirconia solid electrolyte oxygen sensor with a Bi/Bi 2 O 3 reference electrode, focusing on its output accuracy. When the above sensor was tested in a flow of gas with known oxygen partial pressure, p O2 , a linear relationship between lgp O2 and the electromotive force (EMF) was observed, and the correlation slope exhibited a positive deviation from Nernstian behavior. EMF measurements performed in molten NaCl–KCl indicated that the oxygen content of this salt mixture increased with increasing oxygen partial pressure in the covering gas, in agreement with Henry’s law. Moreover, the EMF exhibited a linear decrease with increasing melt temperature of molten NaCl–KCl, in agreement with the theoretical model. Finally, a relationship between the structure of molten NaCl–KCl and its oxygen diffusion behavior was established. As a result, the developed sensor was demonstrated to be well suited for determining the oxygen content of molten salts.