oxygen is released as a waste product by plants during the process of

Oxygen is released as a waste product of plants during the process of Photosynthesis.

Photosynthesis is a biological process that occurs in plant cells and other organisms, by which the sun's solar energy is converted into chemical energy that serves as a food for the plants or organisms. In plants, the light energy is captured by the chlorophyll, which is also responsible for its green pigmentation, hence light energy is then transformed into nutrients which are essential for the growth and development of a plant.

The chemical formula for photosynthesis is represented by:
6CO2 + 6H2O + Sun's Solar Energy → 6O2 + C6H12O6
read as, 
Six carbon dioxide molecules, plus six water molecules plus the sun's solar energy will yield six oxygen molecules and glucose. 

Hence, it can be seen that after the process of photosynthesis, six oxygen molecules is released.

Final answer:

Photosynthesis.