A method and a device for measuring an oxygen content of a headspace gas in a beverage can. The beverage can is oriented upside down to allow the headspace gas to collect at the bottom. A hollow piercer on a piercing head forms a sampling opening in the bottom of the can through which the sampling tube penetrates. The liquid level in the beverage can is lowered to establish a direct connection of the gas-filled headspace and the sampling opening. Then the headspace gas is transported from the headspace to a sensor unit via the sampling tube and/or the hollow piercer or the piercing head. The oxygen content and/or an oxygen partial pressure and/or a headspace volume of the headspace gas are determined by the sensor unit. The sampling opening is closed off airtight by sealing elements arranged on the piercer or the piercing head.