Low Oxygen creates High Carbon Monoxide (CO) when burning propane.
Carbon Monoxide can occur in high Oxygen when Carbon Monoxide is leaking in from somewhere else.
Therefore, a Carbon Monoxide (CO) detector is the most important detector to have, as it will also catch low Oxygen.
As propane burns, part of its waste is Carbon DiOxide (CO2) - harmless in low quantities. As Oxygen is depleted, there is not enough Oxygen (low oxygen) to combine with the Carbon to make CO2 in the propane burning waste, so more and more waste turns into poisonous Carbon Monoxide (CO). It is an avalance of poisonous Carbon Monoxide from there as the amount increases rapidly as more Oxygen is depleted.