Matty warns Mac that he has to hurry because the air in the chamber is 78% nitrogen and it will kill him. Air is normally 78% nitrogen.
The doctor states that the O2 mix had been diluted and the children would begin suffocating which is incorrect, as they would still be able to breath but not receive enough oxygen. This would correctly be stated as asphyxiation, not suffocation.
Oxygen can't be ignited or exploded by a spark as mentioned here repeatedly. The only danger would be from the high pressure if the vessel is overheated or destabilized.
When Dr. Nakani hands the oxygen cylinder to MacGyver, she tells him there is about three minutes of oxygen remaining in it. In a pressurized environment oxygen is consumed faster, so it's likely he had far less time than three minutes.