We present for the first time phase-resolved UV spectroscopy of an Of ? p star , namely , HD 191612 . The observations were acquired with the Space Telescope Imaging Spectrograph ( STIS ) on-board the Hubble Space Telescope ( HST ) . We report the variability observed in the main photospheric and wind features and compare the results with previous findings for the Of ? p star HD 108 . We show that UV line strengths , H \alpha , and longitudinal magnetic field , vary coherently according to the rotational period ( P _ { rot } = 537.6d ) , providing additional support for the magnetic oblique rotator scenario . The stellar and wind parameters of HD 191612 are obtained based on NLTE expanding atmosphere models . The peculiar wind line profile variations revealed by the new STIS data - not reproduced by 1D atmosphere models - are addressed through non-spherical MHD simulations coupled with radiative transfer . The basic aspects of the UV variability observed are explained and the structure of the dynamical magnetosphere of HD 191612 is discussed .