  We study the influence of X-rays on the wind structure of selected O stars.
For this purpose we use our non-local thermodynamic equilibrium (NLTE) wind
code with inclusion of additional artificial source of X-rays, assumed to
originate in the wind shocks.
  We show that the influence of shock X-ray emission on wind mass-loss rate is
relatively small. Wind terminal velocity may be slightly influenced by the
presence of strong X-ray sources, especially for stars cooler than Teff < 35
000 K.
  We discuss the origin of the Lx/L \sim 10^-7 relation. For stars with thick
wind this relation can be explained assuming that the cooling time depends on
wind density. Stars with optically thin winds exhibiting the "weak wind
problem" display enhanced X-ray emission which may be connected with large
shock cooling length. We propose that this effect can explain the "weak wind
problem".
  Inclusion of X-rays leads to a better agreement of the model ionization
structure with observations. However, we do not found any significant influence
of X-rays on Pv ionization fraction implying that the presence of X-rays cannot
explain the Pv problem.
  We study the implications of modified ionization equilibrium due to shock
emission on the line transfer in the X-ray region. We conclude that the X-ray
line profiles of helium-like ions may be affected by the line absorption within
the cool wind.
