The present description relates to an olefin block copolymer with excellences in elasticity, heat resistance, and processability. The olefin block copolymer comprises a plurality of blocks or segments, each comprising an ethylene or propylene repeating unit and an α-olefin repeating unit at different mole fractions. The olefin block copolymer comprises polymer chains containing 20 to 100 short-chain branches (SCBs) per 1,000 carbon atoms. A point where a first derivative of the number Y of the SCBs per 1,000 carbon atoms of each polymer chain with respect to the molecular weight X of the polymer chain is 0 exists between the minimum and maximum values of the molecular weight X.