Aims. The aim of this paper is the dynamical determination of the quadrupole mass moment J2 of the main-sequence HD 209458 star. Methods. The adopted method is the confrontation between the measured orbital period of its transiting planet Osiris and a model of it. Osiris is assumed to move along an equatorial and circular orbit. Results. Our determination, for given values of the stellar mass M and radius R and by assuming the validity of general relativity, is J2 = (3.5± 0.5)× 10−5. Previous fiducial evaluations based on indirect, spectroscopic measurements yielded, instead, J2 ∼ 10−6: such a value is incompatible with general relativity, given the present-day level of accuracy in measuring the Osiris’ orbital period.