  We study the process $B^- \to K^- \chi_{c0}$ considering intermediate charmed
meson rescattering effects. For this decay mode the naive factorization ansatz
would predict a vanishing amplitude. We estimate contributions from the
$D^{(*)}_{s} D^{(*)}\to K^- \chi_{c0}$ rescattering amplitudes, and compare the
result with recent experimental measurements. We find that rescattering effects
are able to produce a large branching ratio consistent with measurements by
Belle Collaboration. We also consider rescattering effects in $B^- \to K^-
J/\psi$, arguing that they play a similar role in producing a large branching
fraction for this colour-suppressed decay mode.
