Oxygen levels are decreasing globally due to fossil-fuel burning. The changes are too small to have an impact on human health, but the change is measureable. These plots show changes in the atmospheric O2/N2 ratio compared to the ratio in 1985, in units per million (or 0.0001 %).
The O2/N2 ratio can vary due to changes in either O2 or N2. Natural sources and sinks of N2 are much smaller than those of O2.
Data from Scripps Global Oxygen Measurements.