Abstract: An immersible probe for determining the oxygen content of a steel bath, having a disposable tip that contains aluminum, calcium, magnesium, titanium, zirconium or a rare earth metal that reacts exothermically with the bath oxygen to cause a temperature rise whose magnitude is proportional to bath oxygen content. In preferred embodiments, the probe tip captures a predetermined volume of the molten metal in a chamber having a closure of the metal of the bath which accordingly melts and exposes the chamber when the probe tip has penetrated beneath the slag. Plural chambers, one with oxidizable metal and one without, may be provided for precise comparison.