  Chandra gratings spectra of a sample of 15 massive OB stars were analyzed
under the basic assumption that the X-ray emission is produced in an ensemble
of shocks formed in the winds driven by these objects. Shocks develop either as
a result of radiation-driven instabilities or due to confinement of the wind by
relatively strong magnetic field, and since they are radiative, a simple model
of their X-ray emission was developed that allows a direct comparison with
observations. According to our model, the shock structures (clumps, complete or
fractional shells) eventually become `cold' clouds in the X-ray sky of the
star. As a result, it is expected that for large covering factors of the hot
clumps, there is a high probability for X-ray absorption by the `cold' clouds,
resulting in blue-shifted spectral lines. Our analysis has revealed that such a
correlation indeed exists for the considered sample of OB stars. As to the
temperature characteristics of the X-ray emission plasma, the studied OB stars
fall in two groups: (i) one with plasma temperature limited to 0.1-0.4 keV;
(ii) the other wtih X-rays produced in plasmas at considerably higher
temperatures. We argue that the two groups correspond to different mechanisms
for the origin of X-rays: in radiative-driven instability shocks and in
magnetically-confined wind shocks, respectively.
