Technically, I believe the problem is the CO concentration and exposure time, not displacement of oxygen. CO binds to hemoglobin more effectively than oxygen, preventing enough oxygen from getting to the body's tissues. CO poisoning is common: 20,000 hospital visits per year in the United States, and in many countries is the most common form of poisoning. [1]

[1] https://en.wikipedia.org/wiki/Carbon_monoxide_poisoning