The only way this would occur in our lifetime is if there is some impact from a comet or asteroid that burns up a huge area of the Earth’s surface, and then what is likely to happen is that plants won’t grow, but we’ll be ok, left to freeze to death, starve to death or resort to cannibalism. Plants and algae in the ocean are the earth’s lungs. As long as they are well enough, they will keep the air at a sustainable level.
The earth’s oxygen level has changed dramatically since there has been life on the planet. Remember the evidence of the giant flying insects? Those wonderful dragonflies? Their existence was made possible by a very oxygen enriched atmosphere back millions of years ago. We live now with much less oxygen than there was then. If the decline is gradual enough for evolution to adapt, then @worriedguy ‘s scenario of a race of humans adapted for their environment may come to pass, otherwise, we go the way of the dinosaur.
The other, long term situation is that the sun will go through phases that will cause more radiation on Earth, burning out the atmosphere and destroying our life-giving bubble. But scientists estimate that is millions and millions of years away.