A systematic study on the geometric and electronic structures of ground-state silicon oxide clusters (SinOm, where n, m = 1-8) has been performed using molecular orbital and density functional theories. We find that most of the structures contain planar or buckled ring units. Pendent silicon atoms bonded only to a single oxygen atom are found in silicon-rich clusters. Oxygen-rich clusters have perpendicular planar rings, while silicon monoxide like clusters usually form a large buckled ring. Structures made up of tetrahedrally bonded units are found only in two clusters, Furthermore, the energy gap and net charge distribution for clusters with different silicon:oxygen ratios have been calculated.