X-ray surveys carried out with the Einstein Observatory (Chlebowski et al. 1989) and ROSAT (Berghöfer et al. 1996) have shown that all O stars are soft X-ray emitters. Since O star winds are opaque at soft X-ray energies the stars or their photospheres cannot be the origin of the observed X-ray emission, thus, this emission must be produced in their stellar winds. Obviously, the X-ray emission is connected to dynamical processes present in the winds of O stars; steady-state computations for O star winds which are able to explain many of the observational features cannot predict any X-ray emission.
Lucy & White (1980) suggested the presence of hot gas in the stellar winds which is produced in shocks developing from the growth of instabilities in the winds; supersonic wind flows in O stars are known to be intrinsically unstable. Numerical simulations confirmed this scenario. However, so far these simulations are limited to one or two dimensions and are not able to explain for instance the observed X-ray luminosity of O stars.