The chemical formula of water is H2O, and the chemical formula of hydrogen peroxide is H2O2. It can be known that the molecule of hydrogen peroxide has one more O atom than the molecule of water.

The chemical formula of sodium oxide is Na2O, and the chemical formula of sodium peroxide is Na2O2. It can be known that sodium peroxide molecules have one more oxygen atom than sodium oxide molecules.

It can be judged that the molecular composition of barium peroxide is one o atom more than that of barium oxide BaO, and the chemical formula of barium peroxide can be expressed as BaO 2；;

So choose B.