  A number of O stars are suspected to have (weak) magnetic fields because of
the observed cyclical variability in their UV wind-lines. However, direct
detections of these magnetic fields with optical spectropolarimetry have proven
to be very difficult. We have searched for non-thermal radio emission, which
would be a strong indication for the presence of a magnetic field. Of our 5
selected candidate magnetic O stars, 3 are detected: $\xi$ Per, which we find
to have a non-thermal spectrum, and $\lambda$ Cep and $\alpha$ Cam which show a
thermal spectrum. We also find that the fluxes are lower than the expected
free-free (thermal) contribution of the stellar wind. This is in agreement with
recent findings that the mass-loss rates from O stars using H$\alpha$ are
overestimated because of clumping in the inner part of the stellar wind.
