Are we evolving as fast as the oxygen is depleting?
Good estimates from ice core samples put the decrease in oxygen concentration at 0.7% over the last 800,000 years. At sea level, currently, inspired $P_{O2}$ is (760 mm Hg - 47) * 0.21, approximately 150mm Hg. Human physiology easily adapts to lower $P_{O2}$ levels, with approximately 140 million people living at elevations over 2500 m, with inspired $P_{O2}$ at 108mm Hg, and many permanent human settlements higher than 5000 m, with inspired $P_{O2}$ approximately half what it is at sea level (see West Respiratory Physiology, Ch. 9). A 0.7% decline over 800,000 years isn't going to be much of a challenge.