Believe me, that's soooooo much easier than measuring the gas pressure inside any other way. That's the partial pressure of oxygen. If you have not already done so, complete the Sample and Practice exercises 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 10.10, and 10.11 . Be able to derive the equation for molecular weight of a gas from the ideal gas equation. And they'll sink in a room with lighter gases present.
Water always occupies some of the pressure in a gas collected over water.
O is strictly a function of temperature.