  We have observed several cloud cores in the Orion B (L1630) molecular cloud
in the 2-1 transitions of C18O, C17O and 13C18O. We use these data to show that
a model where the cores consist of very optically thick C18O clumps cannot
explain their relative intensities. There is strong evidence that the C18O is
not very optically thick. The CO emission is compared to previous observations
of dust continuum emission to deduce apparent molecular abundances. The
abundance values depend somewhat on the temperature but relative to `normal
abundance' values, the CO appears to be depleted by about a factor of 10 at the
core positions. CO condensation on dust grains provides a natural explanation
for the apparent depletion both through gas-phase depletion of CO, and through
a possible increase in dust emissivity in the cores. The high brightness of
HCO+ relative to CO is then naturally accounted for by time-dependent
interstellar chemistry starting from `evolved' initial conditions. Theoretical
work has shown that condensation of H2O, which destroys HCO+, would allow the
HCO+ abundance to increase while that of CO is falling.
