When hydrogen is burned in oxygen to form water, the composition of water formed does not depend on the amount of oxygen reacted. interpret this in terms of the law of definite proportion?

Law of Definite Proportions states that a compound is composed of the same ratio of elements present. The formation of water has a reaction:

H2 + 1/2 O2 = H2O

The limiting reactant would be the element with the least ratio of molar mass to stoichiometric coefficient, and that would be H2, not O2.