The process of conversion of inorganic substance into organic substance in presence of light by autotrophs is known as photosynthesis
Autotrophs : the organism which can synthesize their own food
The inorganic substanc used by autotrophs is CO2 and water
This CO2 and water is converted to glucose in presence of light or chemical energy.
The reaction is
6CO2 + 6H2O ---> C6H12O6 + 6O2
Thus in the process O2 is released as by product