What keeps atmospheric oxygen and carbon dioxide at stable levels?

1) The level of Carbon dioxide grows with the emission of CO2 by humans, animals and by human activity but it's kept stable by the process of photosynthesis, for which plants need carbon dioxide, so they consume it

2) Oxygen is consumed by humans and animals, so its level would go down, but again: plants create new oxygen through photosynthesis

So the answer is: plants that undergo photosynthesis