  Recent observations have revealed the existence of a population of
slowly-rotating, nitrogen-rich B dwarfs that are not predicted by evolutionary
models including rotational mixing. However, as theoretical arguments suggest
that magnetic processes may significantly increase the efficiency of the
transport of the chemical elements, it is of importance to assess the extent of
mixing in some known magnetic OB stars. We review our knowledge of the CNO
abundance properties of these objects and present the first results of an NLTE
abundance study of massive stars identified as being magnetic by the MiMeS
collaboration. Although a nitrogen excess is often associated with the presence
of a magnetic field, there is no evidence for a strict one-to-one
correspondence between these two phenomena. This therefore suggests that other
(still elusive) parameters may control the amount of mixing experienced by
main-sequence OB stars.
