  The presence of magnetic fields in O-type stars has been suspected for a long
time. The discovery of such fields would explain a wide range of well
documented enigmatic phenomena in massive stars, in particular cyclical wind
variability, H alpha emission variations, chemical peculiarity, narrow X-ray
emission lines and non-thermal radio/X-ray emission.
  To investigate the incidence of magnetic fields in O stars, we acquired 38
new spectropolarimetric observations with FORS1 (FOcal Reducer low dispersion
Spectrograph) mounted on the 8-m Kueyen telescope of the VLT.
  Spectropolarimetric observations have been obtained at different phases for a
sample of 13 O stars. 10 stars were observed in the spectral range 348-589nm,
HD36879 and HD148937 were observed in the spectral region 325-621nm, and
HD155806 was observed in both settings. To prove the feasibility of the FORS1
spectropolarimetric mode for the measurements of magnetic fields in hot stars,
we present in addition 12 FORS1 observations of the mean longitudinal magnetic
field in theta^1 Ori C and compare them with measurements obtained with the
MuSiCoS, ESPaDOnS and Narval spectropolarimeters.
  Most stars in our sample which have been observed on different nights show a
change of the magnetic field polarity, but a field at a significance level of
3sigma has been detected only in four stars, HD36879, HD148937, HD152408, and
HD164794. The largest longitudinal magnetic field, <B_z> = -276+-88G, was
detected in the Of?p star HD148937. We conclude that large-scale organised
magnetic fields with polar field strengths larger than 1kG are not widespread
among O-type stars.
