Apparatus for the continuous measurement of oxygen contained in a molten metal which comprises A. A SOLID OXYGEN ELECTROLYTE TUBE; B. A REFERENCE ELECTRODE HAVING KNOWN OXYGEN POTENTIAL IN CONTACT WITH THE INNER SURFACE OF THE TUBE; C. MEANS FOR MEASURING THE ELECTROCHEMICAL POTENTIAL PRESENT BETWEEN THE INNER AND OUTER SURFACES OF THE TUBE; AND D. INSULATING MEANS TO PROTECT THE SOLID OXYGEN ELECTROLYTE TUBE AGAINST CHEMICAL ATTACKS, THERMAL SHOCKS AND MECHANICAL STRESS DUE TO THE CONTACT WITH THE LIQUID MOLTEN METAL. The insulating means is a tube made of a metal containing substance having a high resistance to mechanical stress, to thermal shocks, to chemical attack, to splashing of the metal which is to be tested and to the slag of the metal which is to be tested. In addition the insulating means closely surrounds the solid oxygen electrolyte tube and is open at the bottom so that the outer surface of the tube can be directly contacted with the molten metal which is to be tested.