Sodium, like all the elements on that column have one valence electron. Valence electrons are the electrons found at the most outer shell of an atom. These are the electrons that do all the chemistry stuff. Oxygen on the other hand has six valence electrons. Adding just one sodium atom to an oxygen atom won’t create a stable bond( eight valence electrons). Sodium needs to lose one electron while oxygen needs to gain two. Therefore, by adding two sodiums( each transfers one electron to oxygen) to one oxygen, you get a stable bond and Sodium Oxide. I hope that this helps.