We report the effects of oxygen on the nucleation, growth and relaxation of Ag nanostructures on Ag(100). Comparisons with previous observations on the clean, oxygen-free Ag(100) surface, provide an indirect method to determine how oxygen affects the atomic-scale diffusional processes on the surface. Experiments were performed in UHV using High-Resolution Low Energy Electron Diffraction and Variable Temperature Scanning Tunneling Microscopy. Oxygen exposures were performed prior-to, during, and after the deposition of Ag at temperatures usually ≤250 K.;Experimental data show that the mechanism for submonolayer-island coarsening changes from island diffusion/coalescence to Ostwald ripening after exposure to oxygen. The change in mechanism is manifest in an increase in the rate for island coarsening. Exposure to oxygen also enhances the rate for smoothing of mounded multilayer films. Decay analysis of multilayer island stacks reveals that oxygen adatoms (Oad) are mobile on the Ag terraces, and are able to freely attach/detach from terrace steps. It is speculated that O ad aids in the detachment of AgnO (n = 1,2) from the surface steps, driving the coarsening process. After extended island evolution, Ag islands undergo a change in size and shape, with island and terrace steps adopting the less favorable (open) [100] orientation. The change in surface step geometry is a result of oxygen-induced formation and stabilization of kinks at the surface steps.;We also show that the presence of Oad prior to the deposition of Ag results in the decrease in the initial island density of surface islands. This implies that mobile Oad and/or AgnO interferes with Ag deposition. It is also possible that incorporation of oxygen destabilizes small Ag clusters, causing them to break apart, or makes the entire cluster itself mobile. Further analysis through multiple-step deposition experiments verifies the existence of mobile O-species. The change in island density demonstrates that the pre-exposure of the Ag surface to oxygen provides an interesting means to control the formation of Ag nanostructures during the deposition process.