  A sample of OB stars (eleven Of, one O and one B supergiant) has been
surveyed with the Australia Telescope Compact Array at 4.8 and 8.64 GHz with a
resolution of 2'' -- 4''. Five stars were detected; three of them have negative
spectral indices, consistent with non-thermal emission, and two have positive
indices. The thermal radiation from HD 150135 and HD 163181 can be explained as
coming from an optically thick ionized stellar wind. The non-thermal radiation
from CD-47 4551, HD 124314 and HD 150136 possibly comes from strong shocks in
the wind itself and/or in the wind colliding region if the stars have a massive
early-type companion. The percentage of non-thermal emitters among detected O
stars has increased up to ~50%. The Of star HD 124314 clearly shows flux
density variations. Mass loss rates (or upper limits) were derived for all the
observed stars and the results compared with non-radio measurements and
theoretical predictions.
