Calculating the amount of oxygen in a glass using candle burning time
I think you will be better off measuring the volume change after complete burning.
A common demonstration  was to place a candle in a dish of shallow water and light it, then put a clear cup over it and see how much water was sucked into the bottom as the oxygen is consumed. utilizing the same effect in a more controlled way perhaps would be a straightforward way to get a rough oxygen concentration.
When calculating it remember that 2 oxygen molecules are replaced by a single co2 molecule and you will want your water deep enough so that when the heated air expands it doesn't escape out the bottom and account for temperature differences either by doing the math or waiting for the temp to be the same.
Fully burning steel wool and measuring the gain in weight due to the oxygen getting incorporated into solid iron oxides might also work.