Isotope effect is a strong experimental tool which is used to investigate complex systems. Ideally, the isotope substitution affects only one parameter, for example, a phonon frequency which is directly related to the nuclear mass. However, in cuprate superconductors the complexity of the materials limits the ability to interpret isotope experiments since many parameters are strongly dependent. The isotope effect in another fundamental cuprates phase, the antiferromagnetism, may help resolve this problem. We report meticulous measurements in which we observe the absence of oxygen isotope effect on $T_N$ with unprecedented accuracy. This finding, analyzed together with previous results, suggests that isotope substitution affects mainly the normal charge carriers density.