  We conduct the first 3D hydrodynamic simulations of oxygen-neon white
dwarf-neutron star/black hole mergers (ONe WD-NS/BH mergers). Such mergers
constitute a significant fraction, and may even dominate, the inspiral rates of
all WD-NS binaries. We post-process our simulations to obtain the nuclear
evolution of these systems and couple the results to a supernova spectral
synthesis code to obtain the first light curves and spectra for these
transients. We find that the amount of $^{56}$Ni synthesised in these mergers
grows as a strong function of the WD mass, reaching typically $0.05$ and up to
$0.1\,{\rm M}_\odot$ per merger. Photodisintegration leads to similar amounts
of $^4$He and about a ten times smaller amount of $^1$H. The nuclear yields
from these mergers, in particular those of $^{55}$Mn, may contribute
significantly to Galactic chemical evolution. The transients expected from ONe
WD-NS mergers are dominantly red/infrared, evolve on month-long timescales and
reach bolometric magnitudes of up to -16.5. The current surveys must have
already detected these transients or are, alternatively, putting strong
constraints on merger scenarios. The properties of the expected transients from
WD-NS mergers best agree with faint type Iax supernovae. The Vera Rubin
Observatory (LSST) will be detecting up to thousands of merging ONe WD-NS
systems per year. We simulate a subset of our models with 2D axisymmetric FLASH
code to investigate why they have been challenging for previous studies. We
find that the likely main challenge has been effectively modelling the nuclear
statistical equilibrium regime in such mergers.
