Hydraulic Oil also called hydraulic liquids, hydraulic fluids are the medium by which power is transferred in hydraulic machinery. Common hydraulic oil are based on mineral oil or water. Examples of equipment that might use hydraulic oil include excavators, brakes, power steering systems, transmissions, backhoes, garbage trucks, aircraft flight control systems and industrial machinery. There are other important functions of hydraulic oil such as protection of the hydraulic machine components.

Environmentally sensitive applications may benefit from using biodegradable hydraulic oil / hydraulic fluids based upon rapeseed vegetable oil when there is the risk of an oil spill from a ruptured oil line.