C-H...O hydrogen bonding interactions in crystalline organometallic complexes and clusters have been investigated. The analysis takes the form of retrieval from the Cambridge Structural Database of all intra- and intermolecular H...O distances less than 2.80 Angstrom, associated with appropriate C-H...O angular geometry, in organometallic crystal structures of the first-row transition elements containing terminal and bridging CO ligands. Individual crystal structures have also been examined to evaluate the role of C-H...O hydrogen bonds in determining the supramolecular arrangement. There is a definite manifestation of C-H...O hydrogen bonds in determining the supramolecular arrangement. There is a definite manifestation of C-H...O hydrogen bonding in this group of crystalline substances. In general, the bridging CO ligands form shorter and more linear hydrogen bonds than do the terminal CO ligands, reflective of their higher basicity. This effect is especially pronounced for intermolecular hydrogen bonds, the intramolecular ones being expectedly more dependent on the details of the molecular structure. C-H...O hydrogen bonds formed by both bridging and terminal CO ligands are also directional. In both cases, there is a tendency for the CO...H angle to be around 140 degrees, hinting that there is oxygen lone pair density in these ketonic directions. Examination of specific structures shows that bifurcated acceptors (several C-H groups pointing toward a single O atom) are common and that the anisotropic displacement parameters of atoms involved in C-H...O bonding are smaller than those of other atoms. It is suggested that the C-H...O hydrogen bond is an example of a soft intermolecular interaction as compared to the hard O-H...O hydrogen bond.