Transitions associated with the vibrations ν1, ν1 + νb1, ν1 + ν51, and ν1 + ν51 – ν51 of the complex OC···Cl2 have been rovibrationally analyzed for several isotopologues involving isotopic substitutions in Cl2. Spectra were recorded using a recently constructed near-infrared (4.34 to 4.56 μm), quantum-cascade laser spectrometer with cw supersonic slit jet expansion. Spectral analysis allowed precise determination of the ν51 intermolecular vibration of OC–35Cl2 to be 25.977637(80) cm–1. These results were incorporated with other previously determined data into a spectroscopic database for generation of a five-dimensional morphed potential energy surface. This compound-model morphed potential with radial shifting (CMM-RS) was then used to make more accurate predictions of properties of the OC–35Cl2 complex including De = 544(5) cm–1, D0 = 397(5) cm–1, ν3 = 56.43(4) cm–1, and νb1 = 85.43(4) cm–1. The CMM-RS potential determined for OC–Cl2 was also used to compare quantitatively many of the inherent properties of this non-covalent halogen bonded complex with those of the closely related hydrogen-bonded complex OC–HCl, which has a similar dissociation energy D0. We found that in the ground state, the CO bending amplitude is larger in OC–Cl2 than in OC–HCl.