BaO is the oxide of the feebly acidic ation Ba²⁺. This oxide is a basic oxide. OsO₄ is the oxide of Os⁸⁺ which would be extremely acidic. This oxide is an acidic oxide. The product of an acidic oxide and a basic oxide will yield a salt ("neutralization"). Osmium is a relatively large atom so its oxo anion would be predicted to have six oxygens, OsO₆^4-. Two Ba²⁺ cations are needed to balance the -4 charge of the oxo anion.

BaO + OsO₄ Ba₂OsO₆