  Interactions between convective shells in evolved massive stars have been
linked to supernova impostors, to the production of the odd-Z elements Cl, K,
and Sc, and they might also help generate the large-scale asphericities that
are known to facilitate shock revival in supernova explosion models. We
investigate the process of ingestion of C-shell material into a convective
O-burning shell, including the hydrodynamic feedback from the nuclear burning
of the ingested material. Our 3D hydrodynamic simulations span almost 3 dex in
the total luminosity $L_\mathrm{tot}$. All but one of the simulations reach a
quasi-stationary state with the entrainment rate and convective velocity
proportional to $L_\mathrm{tot}$ and $L_\mathrm{tot}^{1/3}$, respectively.
Carbon burning provides $14\,$--$\,33\%$ of the total luminosity, depending on
the set of reactions considered. Equivalent simulations done on $768^3$ and
$1152^3$ grids are in excellent quantitative agreement. The flow is dominated
by a few large-scale convective cells. An instability leading to large-scale
oscillations with Mach numbers in excess of $0.2$ develops in an experimental
run with the energy yield from C burning increased by a factor of 10. This run
represents most closely the conditions expected in a violent O-C shell merger,
which is a potential production site for odd-Z elements such as K and Sc and
which may seed asymmetries in the supernova progenitor. 1D simulations may
underestimate the energy generation from the burning of ingested material by as
much as a factor two owing to their missing the effect of clumpiness of
entrained material on the nuclear reaction rate.
