  Although significant progress has been achieved in recent surveys of the
magnetism in massive stars, the origin of the detected magnetic fields remains
to be the least understood topic in their studies. We present an analysis of 61
high-resolution spectropolarimetric observations of 36 systems with O-type
primaries, among them ten known particle-accelerating colliding-wind binaries
exhibiting synchrotron radio emission. Our sample consists of multiple systems
with components at different evolutionary stages with wide and tight orbits and
different types of interactions. For the treatment of the complex composite
spectra of the multiple systems, we used a special procedure involving
different line masks populated for each element separately. Out of the 36
systems, 22 exhibit in their LSD Stokes V profiles definitely detected Zeeman
features, among them seven systems with colliding winds. For fourteen systems
the detected Zeeman features are most likely associated with O-type components
whereas for three systems we suggest an association with an early B-type
component. For the remaining five systems the source of the field is unclear.
Marginal evidence for the detection of a Zeeman feature is reported for eleven
systems and non-detection for three systems. The large number of systems with
definitely detected Zeeman features presents a mystery, but probably indicates
that multiplicity plays a definite role in the generation of magnetic fields in
massive stars. The newly found magnetic systems are supreme candidates for
spectropolarimetric monitoring over their orbital and rotation periods to
obtain trustworthy statistics on the magnetic field geometry and the
distribution of field strength.
