Are trees world's lungs? In a way!

Very simplified, but rather spot on:

A tree takes up CO2 from the air, minerals and water from the ground. With the help of solar energy the tree converts this into sugar and oxygen as long as the sun shines during the day.

Then in the tree uses this sugar plus water, minerals and oxygen for the cellular respiration and the construction of new cells just like any living organism at any time.

Thus the tree is generating oxygen during sun shine and but consumes a portion of it for its own metabolism. Overall more oxygen is generated than consumed, so when the tree reaches maturity, it has contributed to releasing oxygen into the atmosphere.

If it just dies and falls to the ground, it will decay and in the spirit of nature’s cycles, the small insects and micro organisms that feed on the dead tree will consume the small excess of oxygen that the tree produced during its life.

So in a perfect natural eco system, there is no extra oxygen produced. The amount of oxygen in the atmosphere is rather stableIt. But is very important that there are plants or other biomass that can convert the CO2 animals breathe out to the O2 so that the cycle works.