We have performed systematic ab initio calculations to study the structures and stability of Si6On clusters (n = 1-12) in order to understand the oxidation process in silicon systems. Our calculation results show that oxidation pattern of the small silicon cluster, with continuous addition of O atoms, extends from one side to the entire Si cluster. Si atoms are found to be separated from the pure Si cluster one-by-one by insertion of oxygen into the Si- O bonds. From fragmentation energy analyses, it is found that the Si- rich clusters usually dissociate into a smaller pure Si clusters (Si-5, Si-4, Si-3, or Si-2), plus oxide fragments such as SiO, Si2O2, Si3O3, Si3O4, and Si4O5. We have also studied the structures of the ionic Si6On +/- (n = 1-12) clusters and found that most of ionic clusters have different lowest- energy structures in comparison with the neutral clusters. Our calculation results suggest that transformation Si6On+(a) + O -> Si6On+1+(a) should be easier.