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We have performed an unbiased dense core survey toward the Orion A GiantMolecular Cloud in the C$^{18}$O ($J$=1--0) emission line taken with theNobeyama Radio Observatory (NRO) 45-m telescope. The effective angularresolution of the map is 26", which corresponds to $\sim$ 0.05 pc at a distanceof 414 pc. By using the Herschel-Planck H$_2$ column density map, we calculatethe C$^{18}$O fractional abundance and find that it is roughly constant overthe column density range of $\lesssim$ 5 $\times$ 10$^{22}$ cm$^{-3}$, althougha trend of C$^{18}$O depletion is determined toward higher column density.Therefore, C$^{18}$O intensity can follow the cloud structure reasonably well.The mean C$^{18}$O abundance in Orion A is estimated to be5.7$\times$10$^{-7}$, which is about 3 times larger than the fiducial value. Weidentified 746 C$^{18}$O cores with astrodendro and classified 709 cores asstarless cores. We compute the core masses by decomposing the Herschel-Planckdust column density using the relative proportions of the C$^{18}$O integratedintensities of line-of-sight components. Applying this procedure, we attempt toremove the contribution of the background emission, i.e., the ambient gasoutside the cores. Then, we derived mass function for starless cores and foundthat it resembles the stellar initial mass function (IMF). The CMF for starlesscores, $dN/dM$, is fitted with a power-law relation of $M^\alpha$ with a powerindex of $\alpha = -$2.25$\pm$ 0.16 at the high-mass slope ($\gtrsim$ 0.44$M_\odot$). We also found that the ratio of each core mass to the total massintegrated along the line of sight is significantly large. Therefore, in theprevious studies, the core masses derived from the dust image are likely to beoverestimated at least by a factor of a few. Accordingly, such previous studiesmay underestimate the star formation efficiency of individual cores.

The C$^{18}$O core mass function toward Orion A: Single-dish observations