The lungs in our body take oxygen from the air and send it to the whole body.
The other answers to this question are correct about the various and varied sources that generate the oxygen in the atmosphere. But what is missed by these answers is that there is a huge reservoir of oxygen already in the atmosphere such that if all oxygen generation by photosynthesis everywhere on earth is completely stopped right now, life would not be in trouble due to a lack of oxygen for a very long time. Life would run out of food if all photosynthetic life died long before life would run out of oxygen.
I could not find data for the total respiration rate of life on earth, but since respiration is essentially the burning (with oxygen) of the food (fuel) that life consumes, we could instead look at the question of whether we would run out of oxygen if we were to burn all of the biomass on earth. After all, when one organism eats another organism as food, it will eventually "burn" the food organism - so by considering the burning of all organisms we are considering the case where life eats all other life on earth.
Now according to Wolfram Alpha, the total biomass of the earth is only about 1/64,000th of the total atmospheric mass of the earth ( see
http://m.wolframalpha.com/input/... ). Since the atmosphere is 20% oxygen, it is clear that burning all the biomass of the earth would only decrease atmospheric oxygen content by a tiny amount.
That is why short term fluctuations in the rate of oxygen generation in photosynthesis compared to the short term rate of oxygen consumption in respiration does not matter. The huge buffer of oxygen in the atmosphere totally damps out these short term fluctuations - it is only the long term balance of generation and consumption of oxygen which will make any change to the total oxygen content of the atmosphere.