Oxygen levels are decreasing slightly, but there is a lot more oxygen (20.95%) than CO2 (0.02%). If CO2 doubled to 0.04%, without any oxygen being added back, the level would decrease to 20.93%, which is barely noticeable.

Plants (especially algae) convert lots of CO2 back into oxygen, this slows down the rate at which CO2 levels are creeping upwards to a crawl. A change in the natural ecosystem which disturbs photosynthesis, such as a mass die-off of algae, would cause a much larger shift in O2 and CO2 levels.