Think of baking a standard recipe: whether you bake a single cake or a massive batch, the ratio of ingredients (like flour to sugar) remains strictly constant.

Remember 'Water is 1:8 by mass (H:O)' or 'O:H is 16:2'.

Water has a molecular formula of $H_2O$. The atomic mass of hydrogen is $1\text{ u}$ and oxygen is $16\text{ u}$. For $1$ mole of water ($18\text{ g}$), there are $2\text{ moles}$ of hydrogen ($2\text{ g}$) and $1\text{ mole}$ of oxygen ($16\text{ g}$). Therefore, the mass ratio of Oxygen to Hydrogen ($O:H$) is $16:2$.

Always pay careful attention to whether the question asks for (Hydrogen : Oxygen) or (Oxygen : Hydrogen) to avoid inversion errors in ratios.