  We present the first 3D simulation of the last minutes of oxygen shell
burning in an 18 solar mass supernova progenitor up to the onset of core
collapse. A moving inner boundary is used to accurately model the contraction
of the silicon and iron core according to a 1D stellar evolution model with a
self-consistent treatment of core deleptonization and nuclear
quasi-equilibrium. The simulation covers the full solid angle to allow the
emergence of large-scale convective modes. Due to core contraction and the
concomitant acceleration of nuclear burning, the convective Mach number
increases to ~0.1 at collapse, and an l=2 mode emerges shortly before the end
of the simulation. Aside from a growth of the oxygen shell from 0.51 to 0.56
solar masses due to entrainment from the carbon shell, the convective flow is
reasonably well described by mixing length theory, and the dominant scales are
compatible with estimates from linear stability analysis. We deduce that
artificial changes in the physics, such as accelerated core contraction, can
have precarious consequences for the state of convection at collapse. We argue
that scaling laws for the convective velocities and eddy sizes furnish good
estimates for the state of shell convection at collapse and develop a simple
analytic theory for the impact of convective seed perturbations on shock
revival in the ensuing supernova. We predict a reduction of the critical
luminosity for explosion by 12--24% due to seed asphericities for our 3D
progenitor model relative to the case without large seed perturbations.
