Oxygen is popular but unique among diatomic molecules in that it behaves magnetically at low temperature. Under high pressure, however, we expect the insulator-metal transition. This expectation has been suggested by measuring its optical reflectivity[1] under high pressures around 95 GPa (1 Mbar). A new structural transition[2] is considered to be accompanied by metallization. Obviously, the most direct method of detecting metallization is to measure the electrical resistance. We measured the resistance of oxygen at pressures of over 100 GPa[3] and identified the metallic state from a change in the slope of dR/dT. In this paper, we present our findings on the superconducting transition of oxygen under high pressures of around 100 GPa and at temperatures of under 0. 6 K. The superconducting transition is indicated by a drop in resistance. We confirmed this by observing the magnetic field dependence of the drop and by detecting the Meissner demagnetization signal.
