The number of electrons gained, lost or shared so as to complete the octet of electrons in valence shell is called valency.

Valency of silicon : It has electronic configuration → 2, 8, 4. Thus, 4 electrons are shared with other atoms to complete the octet and so its valency = 4

Valency of oxygen : It has electronic configuration → 2, 6. Thus, It will gain 2 electrons to complete its octet. So its valency = 2