The confusion arises from the way we think about the molecular composition of water and how atoms are represented in molecular form. Water (H2O) is composed of two hydrogen atoms and one oxygen atom. So, for every mole of water we have one mole of oxygen atoms. The chemical formula indicates that for every molecule of water, one oxygen atom is present.
When we talk about the mole ratio of oxygen to water as 1:2, it may seem misleading. The ratio refers to the number of moles of O2 gas needed to form water, which is indeed 1 mole of O2 for every 2 moles of H2 that reacts to form 2 moles of H2O. However, once water is formed, we only need to refer to the number of oxygen atoms in the water itself.
Thus, in practical terms, when calculating the number of oxygen atoms in a given mole of water, we consider it straightforwardly as 1 mole of H2O yielding 1 mole of oxygen atoms. It’s that simple when you directly look at how water molecules are structured.