  NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field
strength $\ge$ 20 kG, which is the strongest magnetic field ever measured in an
O-type star. We study the variability of the mean longitudinal magnetic field
$\langle B_z \rangle$ and the mean field modulus $\langle B \rangle$ to obtain
constraints on its field geometry. Only one magnetic pole is observable over
the rotation cycle. The approximately sinusoidal variation of $\langle B_z
\rangle$ and the ratio of the values of the extrema of $\langle B \rangle$
indicate that there is an important component of the field that is dipolar. The
$\langle B_z \rangle$ values measured over the rotation cycle are in the range
from $-0.2$ to 4.5 kG, whereas the values for $\langle B \rangle$ vary between
9 and 12 kG. The $\langle B_z \rangle$ values obtained using the O III
$\lambda$7455 emission line are in the range from 0.4 to 2.3 kG and show a
variability pattern similar to that detected for the absorption lines. The fact
that the phase of the $\langle B_z \rangle$ minimum coincides with the phase of
the $\langle B \rangle$ maximum, indicates that the field structure must
significantly depart from a centred dipole. Further, we discuss the nature of
the observed variable Stokes $V$ profiles corresponding to a longitudinal field
of negative polarity detected in the emission He I lines and present the first
MHD numerical simulations of the gas flow in the magnetosphere of this star.
