A detailed analysis of the changes in the electronic structure of CO when a proton or a positive charge approaches the carbon or the oxygen atom is reported using quantum mechanical ab initio calculations and several methods to analyze the theoretical data. The C-Q bond is shortened by nearly the same amount in HCO+ and QCO(+) compared to free CO, while the nearly identical C-O bond lengths of COH+ and COQ(+) are longer than in CO. H+ and Q(+) have a strong electrostatic effect upon the atom to which they are bonded, which leads to an increased electronegativity of carbon and oxygen, respectively. Inspection of the charge distribution and the natural localized orbitals shows clearly that the shorter C-O distances of HCO+ and QCO(+) and the longer C-O bond lengths of COH+ and COQ(+) are due to the changes in the polarization of the bonding orbitals which are caused by the positive charge of H+ or Q(+) that are bonded to the molecule. The bonding orbitals of CO are polarized toward the more electronegative oxygen end. A proton or a positive charge at carbon attracts electronic charge from the oxygen atom toward the carbon end, which leads to less polarized sigma- and pi-bonds and to a more covalent C-O bond. A positive charge or a proton at the oxygen atom has the opposite effect. The calculated curve of the C-O bond length in MCO+ (M = Li, Cu, Ag, Au) as a function of the M+-CO distance shows that the C-O bond becomes shorter in the beginning when the metal cation approaches the carbon atom. There is a turning point at shorter M+-CO distances where the C-O bond becomes longer again. The charge decomposition analysis shows that the position of the turning point is determined by the onset of the metal(+) --> CO back-donation. A relatively small amount of M+ --> CO back-donation is sufficient to lengthen the CO bond. The turning point for the curve of the C-O bond length as a function of the M+-CO distance occurs at a M+-CO value that is shorter than the equilibrium distance for M = Li and Ag, while it is longer for M = Cu and Au. The trends of the bond strengths and M+-CO interactions are explained with the radii and orbital energies of the valence ns and (n - 1)d orbitals of the transition metals.