Question

Why is carbon so abundant in living things when oxygen is the most abundunt element on Earth

Answer 1

Carbon atoms can bond with one another to make chains and cyclic structures whereas oxygen atoms cannot form chains or cyclic structures.

Carbon atoms are found in compounds that are solids at room temperature whereas oxygen atoms are found in compounds that form gases

Carbon atoms have lower mass than oxygen atoms, which keeps the masses of living things within desirable limits.

Carbon atoms are more stable than oxygen atoms, which allows living things to have more stable compositions.
Source: http://openstudy.com/study#/updates/503a62d5e4b0edee4f0d97d7