The effects of oxygen flooding on the sputter and ionization yield of Si and SiO2 have been investigated. Additional processes in sputtering and ionization may be introduced by oxygen flooding beyond a simple chemical adsorption and knock-on process, leading to a larger reduction in sputter yield and a further enhancement in ionization yield for silicon sputtered from a Si substrate as compared to the values for silicon dioxide. A transient, particularly for the sputter yield, still remains, implying that the transient reduction is not as complete as desired.