(57) The present invention provides a hydraulic oil composition comprising a base oil composed of a mineral oil (A) and/or synthetic oil (B) incorporated with 20 wt% or less, based on the whole composition, of one or more types of additives (C) including viscosity index improver and/or pour point depressant, and having a kinematic viscosity of 1 to 20 mm²/s at 100°C, a viscosity index of 140 or more and a Brookfield viscosity of 2000 mPa•s or less at -40°C, wherein the mineral oil (A) is a deeply dewaxed mineral oil (a) having a kinematic viscosity of 2 to 3 mm²/s at 100°C, pour point of -30°C or less and cloud point of -27.5°C or less, or a mixed mineral oil of the deeply dewaxed mineral oil (a) and a low-viscosity mineral oil (b) having a kinematic viscosity of 1.0 to 6.0 mm²/s at 40°C, and the synthetic oil (B) is a poly-α-olefin (c) having a kinematic viscosity of 1.5 to 45 mm²/s at 100°C, and/or an ester compound (d) having a kinematic viscosity of 1.5 to 20 mm²/s at 100°C and pour point of -30°C or less.