  We present ALMA continuum and molecular line emission maps at $\sim$1 mm of
OH 231.8, a well studied bipolar nebula around an AGB star. The excellent
angular resolution of our maps ($\sim$20 mas) allows us to scrutinise the
central nebular regions of OH 231.8, which hold the clues to unravel how this
iconic object assembled its complex nebular architecture. We report, for the
first time in this object and others of its kind, the discovery of a rotating
circumbinary disk selectively traced by NaCl, KCl, and H$_2$O emission lines.
This represents the first detection of KCl in an oxygen-rich AGB circumstellar
envelope. The rotating disk, of radius $\sim$30 au, lies at the base of a young
bipolar wind traced by SiO and SiS emission, which also presents signs of
rotation at its base. The NaCl equatorial structure is characterised by a mean
rotation velocity of $\sim$4 km s$^{-1}$ and extremely low expansion speeds,
$\sim$3 km s$^{-1}$. The outflow has a predominantly expansive kinematics
characterised by a constant radial velocity gradient of $\sim$65 km s$^{-1}$
arcsec$^{-1}$ at its base. Beyond $r$$\sim$350 au, the gas in the outflow
continues radially flowing at a constant terminal speed of $\sim$16 km
s$^{-1}$. Our continuum maps reveal a spatially resolved dust disk-like
structure perpendicular to the outflow, with the NaCl, KCl and H$_2$O emission
arising from the disk's surface layers. Within the disk, we also identify an
unresolved point continuum source, which likely represents the central
Mira-type star QX Pup enshrouded by a $\sim$3 $R_{\star}$ component of hot
($\sim$1400 K) freshly formed dust. The point source is slightly off-centered
from the disk centroid, enabling us for the first time to place constraints to
the orbital separation and period of the central binary system, $a$$\sim$20 au
and $P_{\rm orb}$$\sim$55 yr, respectively. (abridged).
