First, we write an equation for the reaction of Hydrogen and Oxygen to produce water:
H₂ + O₂ ⇒ H₂O
This equation is not balanced, we then balance the Oxygens:
H₂ + O₂ ⇒ 2H₂O
And now the Hydrogens:
2H₂ + O₂ ⇒ 2H₂O
Therefore, if one mole of Oxygen reacts, it will require two moles of Hydrogen and produce two moles of water.