The oxygen to hydrogen mass ratio in H₂O₂ (hydrogen peroxide) is 16:1.

Here's how to calculate it:

* Molecular weight of H₂O₂: 2(1.008) + 2(16.00) = 34.016 g/mol

* Mass of oxygen in H₂O₂: 2(16.00) = 32.00 g/mol

* Mass of hydrogen in H₂O₂: 2(1.008) = 2.016 g/mol

Therefore, the ratio of oxygen mass to hydrogen mass is 32.00:2.016, which simplifies to 16:1.