The invention relates to a device for uncoupling the oil flow through a radiator (7), in particular a radiator for a transmission with low oil temperatures. The device is configured as a bypass valve (1) comprising an oil supply (6) from the radiator (7), a supply (4) that runs via a bypass line (5) and an outflow (8) for the oil. The bypass valve contains a valve unit, which can be displaced inside the valve housing (10) against the force of a compression spring (11) that is connected in series. Said valve unit also comprises a heat sensitive element (2) that is displaced inside the valve housing (10) in accordance with the temperature of the oil, in such a way that when the temperature falls below a threshold value, the oil supply (6) from the radiator (7) is closed and the supply (4) that runs via a bypass line (5) is opened. When the temperature rises above the threshold value, the oil supply (6) from the radiator (7) is opened and the supply (4) that runs via the bypass line (5) is closed. The entire valve unit can be axially displaced against the force of the compression spring (11), once the oil pressure has reached a predefined level, thus opening the supply (4) that runs via the bypass line (5).