The geometric and electronic structures of organosilicon oligomers, nSixL(C=C)y with linear silicon segments and nSixR(C=C)y with cyclic silicon segments, are studied by using the density functional theory. It is found from the calculated results that the extension of the π-conjugation plays an important role in tuning the geometric and electronic structures. The band gap tends to be decreased and the hole injection rate tends to be increased with the elongation of the π-conjugated units. An effective σ-π conjugation occurs between the silanylene moiety and the ethenylene moiety, especially for the polymers with a rather small π-electron moiety. Interestingly, an even/odd effect of the band gap is found. A PSixR(C=C)y has narrower band gap than a corresponding PSixL(C=C)y. These results are in good agreement with the experimental observations.