What might happen if you give such a person high levels of supplemental O2?

Administering high levels of supplemental oxygen to persons with carbon monoxide poisoning may worsen their heart disease. Because carbon monoxide binds loosely to hemoglobin but O2 more firmly, providing elevated quantities of O2 results in the displacement of CO, allowing carbon monoxide to rebind to haemoglobin even tighter and exacerbating hypoxia