If locked in a perfectly sealed room, you could die from CO2 overdose before dying from a lack of oxygen.
This a condition that called hypercapnia, when there is too much carbon dioxide in the blood. When this happens, the body goes into emergency resuscitation mode, with an increased heart rate, increased blood pressure, muscle tremors, and short rapid breathing. This is actually designed as a reflex to get your body to breathe properly while you are sleeping if your breathing is being obstructed.
Hypercapnia is also caused by too much CO2 in the air, or if you are re-breathing the CO2 you already exhaled. This can lead to respiratory acidosis, causing an increase in the acidity of your blood. The symptoms of this are diarrhea, vomiting, headaches, coughing, sleepiness, irregular heart rates, comas, and seizures. Overall, a pretty nasty way to die.
This condition has been induced when people are in closed spaces for long periods of time (e.g. submarines, hyperbaric chambers, spacecraft). When CO2 levels were too high in the Apollo 13 spacecraft, the astronauts had to rig up a CO2 absorber to prevent life-threatening acidosis.