A heat-treatment oil composition contains (A) a first base oil with a kinematic viscosity at 40 degrees C. in a range of 5 mm2/s to 60 mm2/s in an amount of 50 mass % to 95 mass % of a total amount of the composition, (B) a second base oil with a kinematic viscosity at 40 degrees C. of 300 mm2/s or more in an amount of 5 mass % to 50 mass %, and (C) an alpha-olefin copolymer. The heat-treatment oil composition according to the invention can reduce distortion unevenness and hardness unevenness accompanying mass-quenching.