A method of backing up a supply of oxygen in an air separation plant, said method comprising: pumping a liquid oxygen stream composed of an oxygen-rich liquid, resulting from indirect heat exchange between downcoming liquid of a lower pressure column and nitrogen-rich vapor column overhead of a higher pressure column of the air separation plant, to produce a pumped liquid oxygen stream and heating the pumped liquid oxygen stream to produce the supply of the oxygen; during normal operation of the air separation plant: feeding streams of the oxygen-rich liquid to a surge tank and a reserve storage tank of larger volume than the surge tank and situated more remotely from the lower pressure column than the surge tank, at least on an intermittent basis, so that the oxygen rich liquid is accumulated in the surge tank and the oxygen-rich liquid is stored within the reserve storage tank as a backup supply; and heating the pumped liquid oxygen stream within a heat exchanger used in cooling at least a portion of the air to a temperature suitable for the cryogenic rectification thereof within the air separation plant, thereby to produce the supply of the oxygen during the normal operation; and during a transient operation of the air separation plant where the air separation plant has ceased operation: isolating the surge tank so that the surge tank does not receive the oxygen-rich liquid; pumping the liquid oxygen stream from the surge tank to produce the pumped liquid oxygen stream and heating the pumped liquid oxygen stream within an auxiliary vaporizer to produce the supply of the oxygen during the transient operation; and replenishing the surge tank with a back-up stream of the oxygen- rich liquid removed from the reserve storage tank, the back-up stream composed of the back-up supply within the reserve storage tank. pumping a liquid oxygen stream composed of an oxygen-rich liquid, resulting from indirect heat exchange between downcoming liquid of a lower pressure column and nitrogen-rich vapor column overhead of a higher pressure column of the air separation plant, to produce a pumped liquid oxygen stream and heating the pumped liquid oxygen stream to produce the supply of the oxygen; during normal operation of the air separation plant: feeding streams of the oxygen-rich liquid to a surge tank and a reserve storage tank of larger volume than the surge tank and situated more remotely from the lower pressure column than the surge tank, at least on an intermittent basis, so that the oxygen rich liquid is accumulated in the surge tank and the oxygen-rich liquid is stored within the reserve storage tank as a backup supply; and heating the pumped liquid oxygen stream within a heat exchanger used in cooling at least a portion of the air to a temperature suitable for the cryogenic rectification thereof within the air separation plant, thereby to produce the supply of the oxygen during the normal operation; and feeding streams of the oxygen-rich liquid to a surge tank and a reserve storage tank of larger volume than the surge tank and situated more remotely from the lower pressure column than the surge tank, at least on an intermittent basis, so that the oxygen rich liquid is accumulated in the surge tank and the oxygen-rich liquid is stored within the reserve storage tank as a backup supply; and heating the pumped liquid oxygen stream within a heat exchanger used in cooling at least a portion of the air to a temperature suitable for the cryogenic rectification thereof within the air separation plant, thereby to produce the supply of the oxygen during the normal operation; and during a transient operation of the air separation plant where the air separation plant has ceased operation: isolating the surge tank so that the surge tank does not receive the oxygen-rich liquid; pumping the liquid oxygen stream from the surge tank to produce the pumped liquid oxygen stream and heating the pumped liquid oxygen stream within an auxiliary vaporizer to produce the supply of the oxygen during the transient operation; and replenishing the surge tank with a back-up stream of the oxygen- rich liquid removed from the reserve storage tank, the back-up stream composed of the back-up supply within the reserve storage tank. isolating the surge tank so that the surge tank does not receive the oxygen-rich liquid; pumping the liquid oxygen stream from the surge tank to produce the pumped liquid oxygen stream and heating the pumped liquid oxygen stream within an auxiliary vaporizer to produce the supply of the oxygen during the transient operation; and replenishing the surge tank with a back-up stream of the oxygen- rich liquid removed from the reserve storage tank, the back-up stream composed of the back-up supply within the reserve storage tank.