As the dimensions of metal-oxide-semiconductor (MOS) devices keep shrinking, O@sub 2@ molecule is increasingly used as the oxidizing species over H@sub 2@O as it oxidizes silicon more slowly and hence results in better control of film thickness. Here, we use density functional theory to investigate the detailed chemical mechanism of O@sub 2@ reaction with the Si(100)-(2x1) surface using cluster approximations, in which larger clusters are used to examine reactions across dimers as well as to investigate nonlocal effects. Our proposed mechanism confirms the trapping-mediated mechanism previously observed by molecular beam experiments. We find that O@sub 2@(g) initially adsorbs on the "up" silicon atom of the surface dimer with an adsorption energy of 31 kcal/mol. The adsorption is site specific and reaction on the "down" silicon atom is unstable. The adsorbed O@sub 2@(a) then reacts and forms a peroxide bridge structure, which subsequently dissociates and inserts into the dimer bond and the backbond. Reactions involving neighboring dimers also exhibit an adsorbed state in which the O@sub 2@(a) molecule is adsorbed in between the two silicon dimers. In addition to investigating the initial adsorption of oxygen molecules, we also study the atomistic mechanisms leading to the SiO(g) desorption observed at high temperature. The desorption barrier calculated is 65 kcal/mol, which explains the high thermal energy required before SiO(g) desorption occurs.