How many molecules are present in 1 mole of O2? How many molecules are present in 1 mole of H2O?

believe that you asked the wrong question, because clearly 1 mole of O2 is present in 1 mole of H2O2 and 1 mole of O2 is present in 1 mole of O2.

Perhaps you meant to ask “how many molecules are present in 1 mole of H2O2 and how many molecules are present in 1 mole of O2”. In this case:

I’m sure that you remember Avogadro’s Number (by Amadeo Carlo Avogadro, 1776 - 1856), which states that 1 mole of a substance will contain 6.022 x 10^23 molecules.

So, 1 mole of H2O2 contains 6.022 x 10^23 molecules, and 1 mole of O2 contains 6.022 x 10^23 molecules.