The chemisorption sites of molecular oxygen on the technologically important As-rich GaAs(100)c(2x8) surface were imaged with scanning tunneling microscopy (STM). The oxygen atoms insert into the arsenic-gallium backbonds and, subsequently, replace the arsenic atoms in the dimer rows. The displaced arsenic atoms aggregate in clusters of increasing size forming metallic arsenic. The strongly electronegative oxygen atoms are initially attracted by the larger electron density at the arsenic atoms, but the reaction of the oxygen atoms with the gallium atoms is thermodynamically favored. This leads to a 100% chemical selectivity for oxygen insertion into the As-Ga backbonds and subsequent chemisorption of the oxygen atoms into the arsenic sites.