  We have performed C$^{18}$O ($J$=1--0) mapping observations of a
$20'\times20'$ area of the OMC-1 region in the Orion A cloud. We identified 65
C$^{18}$O cores, which have mean radius, velocity width in FWHM, and LTE mass
of 0.18$\pm$0.03 pc, 0.40$\pm$0.15 km s$^{-1}$, and 7.2$\pm$4.5 $M_\odot$,
respectively. All the cores are most likely to be gravitationally bound by
considering the uncertainty in the C$^{18}$O abundance. We derived a C$^{18}$O
core mass function, which shows a power-law-like behavior above 5 $M_\odot$.
The best-fit power-law index of $-2.3\pm0.3$ is consistent with those of the
dense core mass functions and the stellar initial mass function (IMF)
previously derived in the OMC-1 region. This agreement strongly suggests that
the power-law form of the IMF has been already determined at the density of
$\sim10^{3}$ cm$^{-3}$, traced by the C$^{18}$O ($J$=1--0) line.
