The structural and energetic properties of O/Ag(100) surface have been investigated using density functional theory (DFT) in conjunction with the plane wave basis sets. The periodic slab model is employed for the systems containing surface or subsurface 0 atoms at the normal or missing-row Ag(100) surfaces. Three surface cells: (I x 1), (2 x 2) and (2root2 x root2), are employed. The results show that the oxygen atoms at 4-fold hollow site for p(I x 1) surface cell are slightly below the surface silver atoms. And the 0 atoms on the missing-row surface locate at the first silver layer. The hollow site is the most energetically preferred for 0 atomic adsorption on the (100) surface with different coverage. The adsorption energy at the hollow site on the (2root2 x root2) Ag(100) surface with the missing row structure at 0.5 ML is the highest in our studies. The surface and subsurface 0 atoms have different effects on the properties of substrate. The comparison of the 0 atomic adsorption on the three low-index Ag(I 11), Ag(l 00), Ag(l 10) surfaces is also reported.