Is oxygen decreasing if respiration occurs?

Oxygen is consumed in the process of cellular respiration as per the following chemical reaction:

`C_6H_12O_6 + 6 O_2 -> 6 CO_2 + 6 H_2O + ATP`

here, oxygen is used for breakdown of food (in the form of glucose molecule) and energy is generated (in the form of ATP or adenosine triphosphate molecules), along with carbon dioxide and water. 

If respiration was the only process, we would ultimately run out of oxygen. Luckily, we have photosynthesis. During the process of photosynthesis, plants (containing chlorophyll) produce oxygen, according to the following chemical reaction:

`6 CO_2 + 6 H_2O + sunlight -> C_6H_12O_6 + 6 O_2`

In this process, plants consume carbon dioxide and water and (in presence of sunlight) produce glucose and oxygen. Thus, the green plants (with the help of Sun) constantly generate oxygen and help maintain a balance of oxygen in our atmosphere and prevent it from running out.

Hope this helps.