In fact it is DEFINITELY oxygen, for iron and aluminum at least.
Oxidation and Reduction are kinda opposites. In highschool people are taught that Oxidation is a loss of electrons, Reduction is a gain of electrons.
Aluminum comes out of the ground as bauxite, Al2O3. To get pure Aluminum (Al), you need to get rid of the O (oxygen).
This is NOT the case for everything though. Antimony comes out of the ground as Antimonous Sulfide (or something like that)...Sb2S3, and so you need to get rid of the sulfur...