  $\delta$ Orionis is the closest massive multiple stellar system and one of
the brightest members of the Orion OB association. The primary (Aa1) is a
unique evolved O star. In this work, we applied a two-step disentangling method
to a series of spectra in the blue region (430 to 450 nm), and we detected
spectral lines of the secondary (Aa2). For the first time, we were able to
constrain the orbit of the tertiary (Ab) - to 55 450 d or 152 yr - using
variable $\gamma$ velocities and new speckle interferometric measurements,
which have been published in the Washington Double Star Catalogue. In addition,
the Gaia DR3 parallax of the faint component (Ca+Cb) constrains the distance of
the system to (381 $\pm$ 8) pc, which is just in the centre of the Orion OB1b
association, at (382 $\pm$ 1) pc. Consequently, we found that the component
masses according to the three-body model are 17.8, 8.5, and 8.7 M$_{\odot}$,
for Aa1, Aa2, and Ab, respectively, with the uncertainties of the order of $1$
M$_{\odot}$. We used new photometry from the BRITE satellites together with
astrometry, radial velocities, eclipse timings, eclipse duration, spectral line
profiles, and spectral energy distribution to refine radiative properties. The
components, classified as O9.5 II + B2 V + B0 IV, have radii of 13.1, 4.1, and
12.0 R$_{\odot}$, which means that $\delta$ Ori A is a pre-mass-transfer
object. The frequency of 0.478 cycles per day, known from the Fourier analysis
of the residual light curve and X-ray observations, was identified as the
rotation frequency of the tertiary. $\delta$ Ori could be related to other
bright stars in Orion, in particular, $\zeta$ Ori, which has a similar
architecture, or $\varepsilon$ Ori, which is a single supergiant, and possibly
a post-mass-transfer object.
