Is there a balance or feedback between O₂ and CO₂ in the atmosphere?
Yes, increase in CO₂ concentration increases the rate of photosynthesis in plants. The rate of photosynthesis is linealy proportinal to CO₂ concentration in range up to 225ppm  and  slowly increases range 225-650ppm.[Limiting Factors in Photosynthesis, Plant Physiol. (1984) 75, 82-86] (Now CO2 concentration is about 400ppm).
You can read more in Educational article on Nature journal website or in
More scientific review on the topic.