Why is carbon so abundant in living things when oxygen is the most abundant element on earth?

Although oxygen constitutes 21% of the total atmospheric gases utilized by the organisms but carbon is the abundant element of the living organisms.

This happens because a living organism is composed of the smallest structural and functional unit called cell which is formed by the aggregation of the small molecule mostly which contains the carbon.

This is due to the reasons as the carbon element shows two properties called catenation sue to which carbon can form long chains by attaching the same carbon element and tetra-valency where the carbon can attach four different elements.

Thus, carbon is the abundant element of the living organism.