In the reaction, $4{\mathrm{NH}}_3\left(\mathrm{g}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\to 4\mathrm{NO}\left(\mathrm{g}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$, When 1 mole of ammonia and 1 mole of O₂ are made to react to completion, then 

4NH₃ ​+ 5O₂ ​⟶ 4NO + 6H₂​O
1NH₃​ + 1.25O₂ ​⟶ 1NO + 1.5H₂​O
Hence 1 mole of NH₃​ reacts with 1.25 moles of O₂​ to produce 1 mole of NO and 1.5 moles of H₂​O. Hence, when one mole of ammonia and one mole of oxygen are made to react to completion, then all the oxygen is made to react to completion, then all the oxygen is consumed.