The oxygen activity of the ladle slag may be measured in situ with an electrochemical sensor. The measurement principle of this sensor needs further clarification. An experimental and computational study has been carried out of the contribution of specific components to the oxygen activity of the slag and of the factors that influence oxygen activity measurement with the sensor. Experiments on an industrial and on a laboratory scale show that FeOx, SiO2 and MnO exert a measurable influence on the results. The measurements also show a marked influence of the dissolved oxygen content in the steel. The oxygen activity of the slags has been calculated thermodynamically and compared with the sensor measurements. A systematic difference is observed. A hypothesis on the working principle of the sensor and on the cause of occasional problematic measurement values is proposed.