In this chapter, we examine the way oxygen behaves in silicon without any other point defects present. We initially examine isolated Oi, and then an oxygen dimer, O2i. This includes structural and vibrational mode analysis, as well as determination of the migration barrier and path. We then go on to look briefly at higher order oxygen complexes such as the trimer, O3i, and discuss their implications in the development of other defect species such as thermal donors. Since this is still very much work in progress, many of the conclusions are necessarily open ended.