CO2 narcosis. This happens when people are given too much oxygen...this slows down the oxygen driven drive to breathe and we breathe slower... Then CO2 can build up and reach levels that can be toxic...can be lethal as in hypercapnea.  Oxygen bar usage and home oxygen concentrators could pose a problem for people with severe lung problems, but generally it is not an issue. Pilots and astronauts should not have this problem unless they have advanced lung  or neurological disease which is highly unlikely but possible due to the way science presently measures healthy breathing.  A partial answer to this is to develop the breathing and create less dependency on the oxygen. 

"A 68-year-old man with severe dyspnea was admitted as an emergency case. He had no past history of any respiratory or neuromuscular diseases. Immediately after insufflation of oxygen, respiratory arrest occurred. The blood gas analysis showed hypoxemia and severe hypercapnia (PaO2; 32 mmHg, PaCO2; 127 mmHg). We diagnosed as CO2 narcosis, and he was treated with a respirator in the ICU. He showed nonflaccid bilateral diaphragmatic paralysis and muscle atrophy of the upper extremities. As the EMG showed giant spikes of neurogenic pattern, he was diagnosed as ALS. Weaning from the respirator failed because of his respiratory muscle fatigue. He was given rehabilitation during the day time and ventilatory support with the respirator during the night. We conclude that if we meet with an emergency patient with CO2 narcosis without any pulmonary disorder, we have to suspect neuromuscular diseases, e.q. ALS. In some of such cases, mechanical ventilation supports social rehabilitation. more about