Air separation method in which air is separated within cryogenic rectification processes conducted in first and second cryogenic air separation plants. The first cryogenic air separation plant is designed to produce an oxygen-rich product stream and the second cryogenic air separation plant is designed to produce an impure oxygen vapor stream. At least part of the impure oxygen vapor stream is introduced into a lower pressure column of the first cryogenic air separation plant and oxygen contained in such stream along with air separated within the first air separation plant is used in producing the oxygen-rich product stream.