  In this paper we confirm the presence of a globally-ordered, kG-strength
magnetic field in the photosphere of the young O star $\theta^1$~Orionis~C, and
examine the properties of its optical line profile variations. A new series of
high-resolution MuSiCoS Stokes $V$ and $I$ spectra has been acquired which
samples approximately uniformly the rotational cycle of $\theta^1$~Orionis~C.
Using the Least-Squares Deconvolution (LSD) multiline technique, we have
succeeded in detecting variable Stokes $V$ Zeeman signatures associated with
the LSD mean line profile. These signatures have been modeled to determine the
magnetic field geometry. We have furthermore examined the profi le variations
of lines formed in both the wind and photosphere using dynamic spectra. Based
on spectrum synthesis fitting of the LSD profiles, we determine that the polar
strength of the magnetic dipole component is $1150 \la B_{\rm d}\la 1800$~G and
that the magnetic obliquity is $27\degr \la \beta \la 68\degr$, assuming
$i=45\pm 20\degr$. The best-fit values for $i=45\degr$ are $B_{\rm d} = 1300
\pm 150 (1\sigma)$~G and $\beta = 50\degr \pm 6\degr (1\sigma)$. Our data
confirm the previous detection of a magnetic field in this star, and
furthermore demonstrate the sinusoidal variability of the longitudinal field
and accurately determine the phases and intensities of the magnetic extrema.
The analysis of ``photospheric'' and ``wind'' line profile variations supports
previous reports of the optical spectroscopic characteristics, and provides
evidence for infall of material within the magnetic equatorial plane.
