Which organelle uses molecular oxygen to convert and detoxify harmful substances such as alcohol and free radicals:?

The answer is Peroxisomes.

A noteworthy capacity of the peroxisome is the breakdown of long chain unsaturated fats through beta-oxidation. In creature cells, the long unsaturated fats are changed over to medium chain unsaturated fats, which are in this way carried to mitochondria where they are in the long run separated to carbon dioxide and water.