Why don't all living beings die at night due to lack of oxygen?
The rates at which plants consume CO$_2$ and animals consume O$_2$ is minuscule compared to the vast amounts of CO$_2$ and O$_2$ in the atmosphere. The amount of change overnight is not even measurable, unless the plants are in an enclosed structure.

Interestingly, there is a variation in CO$_2$ concentraion over the course of a year as the Northern Hemisphere (where most of the land vegetation is) goes through its seasons. Here is a plot of CO$_2$ over several decades. It shows an annual cycle superimposed on the general upward trend from the accumulation of man-made CO$_2$.



The red trace is the actual data, and the black trace is the time averaged fit to the data.