  Context: Some O stars are suspected to have to have (weak) magnetic fields
because of the observed cyclical variability in their UV wind-lines. However,
direct detections of these magnetic fields using optical spectropolarimetry
have proven to be very difficult. Aims: Non-thermal radio emission in these
objects would most likely be due to synchrotron radiation. As a magnetic field
is required for the production of synchrotron radiation, this would be strong
evidence for the presence of a magnetic field. Such non-thermal emission has
already been observed from the strongly magnetic Ap/Bp stars. Methods: We have
performed 6 & 21 cm observations using the WSRT and use these, in combination
with archival VLA data at 3.6 cm and results from the literature, to study the
radio emission of 5 selected candidate magnetic O stars. Results: Out of our
five targets, we have detected three: $\xi$ Per, which shows a non-thermal
radio spectrum, and $\alpha$ Cam and $\lambda$ Cep, which show no evidence of a
non-thermal spectrum. In general we find that the observed free-free (thermal)
flux of the stellar wind is lower than expected. This is in agreement with
recent findings that the mass-loss rates from O stars as derived from the
H$\alpha$ line are overestimated because of clumping in the inner part of the
stellar wind.
