From the balanced equation we can conclude that
4 moles of Ammonia requires 5 moles of oxygen.
$\therefore$ 1 mole of $NH_3$ requires 1.25 mole of Oxygen
Thus oxygen is required in lesser quantity as compared to Ammonia,
Thus oxygen is the limiting reagent
So when 1 mole of ammonia and 1 mole of oxygen are combined 0.8 moles of ammonia will combine with 1 mole of oxygen and whole of $O_2$ will be consumed
Hence Oxygen is completely consumed.