CO has higher affinity for hemoglobin. The affinity of CO for Hb as compared to O2 is approximately:

CO binds to hemoglobin with an affinity more than 200 times that of oxygen, and this not only directly reduces the oxygen-carrying capacity of blood, but fuher promotes tissue hypoxia by shifting the oxyhemoglobin dissociation curve to the left. CO may also cause harm by other mechanisms including direct disruption of cellular oxidative processes, binding to myoglobin and hepatic cytochromes, and peroxidation of brain lipids. Ref: Bennett M.H., Mitchell S.J. (2012). Chapter e52. Hyperbaric and Diving Medicine. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.