An absolute gas concentration can be measured from the variation in the speed of sound between two gases in principle. Owing to the small difference in the speed of sound between the atmospheric air and oxygen (O2) gas, measuring O2 concentration with high accuracy in the humid atmospheric air using ultrasound needs careful investigation. The authors show successfully a method to measure the absolute concentration of O2 gas in humid atmospheric air using ultrasound. It was possible to measure O2 concentration in the atmospheric air with accuracy by compensating for the influence of temperature and humidity by calculation. The O2 concentration was calculated from the conventional speed of sound equation by utilizing small mass variation for the change in moisture as well as temperature. This method using ultrasound enabled us to measure the O2 concentration in the atmospheric air as 21.0%, which is in agreement with the standard atmospheric dry air. The measurement error values after the humidity compensation are about 0.4% or less. Furthermore, this method takes only about a few ms for measuring O2 concentration and, thus, can be used as a high-speed portable O2 sensor for industrial, environmental, and biomedical instruments.