The invention relates to a probe (100, 200, 300, 400, 500, 600) for the measurement of the oxygen activity of metal melts, in particular steel melts, comprising a reference substance (2) of known oxygen activity in electrically conducting contact (3) with a measuring device; and comprising a solid electrolyte predominantly oxygen ion conducting and negligibly electron conducting at high temperatures and separating the reference substance (2) from the metal melt and having an entry surface (4) for oxygen ions which is in contact with the metal melt, wherein the entry surface (4) of the probe ready for operation is covered by a functional foil arrangement (10,20) in close contact to the entry surface (4).