  We have developed a complete theory for the calculation of the observed
Stokes parameters for radiation emitted from the surface of a rapidly rotating
neutron star (NS) using the oblate Schwarzschild approximation. We accounted
for the rotation of the polarization plane due to relativistic effects along
the path from the stellar surface to the observer. The results were shown to
agree with those obtained by performing full numerical general relativistic
ray-tracing with the \textsc{arcmancer} code. We showed that the obtained
polarization angle (PA) profiles may differ substantially from those derived
for a spherical star. We demonstrated that assuming incorrect shape for the
star can lead to biased constraints for NS parameters when fitting the
polarization data. Using a simplified model, we also made a rough estimate of
how accurately the geometrical parameters of an accreting NS can be determined
using the X-ray polarization measurements of upcoming polarimeters like the
Imaging X-ray Polarimeter Explorer (IXPE) or the enhanced X-ray Timing and
Polarimetry (eXTP) mission.
