  A quite remarkable aspect of non-interacting O-stars with detected surface
magnetic fields is that they all are very slow rotators. This paper uses this
unique property to first demonstrate that the projected rotational speeds of
massive, hot stars, as derived using current standard spectroscopic techniques,
can be severely overestimated when significant "macroturbulent" line-broadening
is present. This may, for example, have consequences for deriving the
statistical distribution of rotation rates in massive-star populations, and for
the use of these rates in stellar evolution models. It is next shown how such
macroturbulence (seemingly a universal feature of hot, massive stars) is
present in all but one of the magnetic O-stars, namely NGC 1624-2. Assuming
then a simple model in which NGC 1624-2's exceptionally strong, large-scale
magnetic field suppresses atmospheric motions down to layers where the magnetic
and gas pressures are comparable, first empirical constraints on the formation
depth of this enigmatic hot-star macroturbulence are derived. The results
suggest an origin in the thin sub-surface convection zone of massive stars,
consistent with a physical origin due to, e.g., stellar pulsations excited by
the convective motions.
