Apparatus for monitoring an oil thermostat arranged in an oil circuit of an internal combustion engine, wherein the oil thermostat is designed to supply an oil flow either to a first flow branch in which an oil cooler is arranged and/or to a bypass flow branch depending on an admission temperature of the oil flow supplied to the oil thermostat, wherein a first outlet of the oil thermostat, which outlet is assigned to the first flow branch, is closed when the admission temperature lies below a first temperature threshold value, wherein the apparatus comprises: a) a sensor device which is designed to determine at least one first parameter by means of which a current desired operation of the oil thermostat can be derived, and to determine at least one second parameter by means of which a current actual operation of the oil thermostat can be derived; and b) an evaluation device which is designed, depending on the first parameter and the second parameter, to identify an occurrence of a malfunction of the oil thermostat, wherein the sensor device is designed to measure the admission temperature of the oil as the first parameter, wherein preferably the oil sump temperature is measured as the admission temperature, characterized a) in that the sensor device is designed to measure a second oil temperature downstream of the oil cooler and upstream of the internal combustion engine and to determine a temperature difference (ΔT) from the admission temperature minus the second oil temperature as the second parameter; and b) in that the evaluation device identifies the occurrence of a malfunction of the oil thermostat, b1) when the admission temperature lies below the first temperature threshold value, at least one predetermined reaction period has elapsed since the admission temperature has been below the first temperature threshold value, and the temperature difference (ΔT) is greater than a first threshold value (ΔT1); and/or b2) when the admission temperature is above the first temperature threshold value at least one predetermined reaction period has elapsed since the admission temperature has been above the first temperature threshold value, and the temperature difference (ΔT) is smaller than a second threshold value (ΔT2); and/or b3) when, after the first temperature threshold value is exceeded by the admission temperature, an increase of the temperature difference (ΔT) proceeds more slowly than a predetermined desired increase of the temperature difference (ΔT). a) a sensor device which is designed to determine at least one first parameter by means of which a current desired operation of the oil thermostat can be derived, and to determine at least one second parameter by means of which a current actual operation of the oil thermostat can be derived; and b) an evaluation device which is designed, depending on the first parameter and the second parameter, to identify an occurrence of a malfunction of the oil thermostat, wherein the sensor device is designed to measure the admission temperature of the oil as the first parameter, wherein preferably the oil sump temperature is measured as the admission temperature, characterized wherein the sensor device is designed to measure the admission temperature of the oil as the first parameter, wherein preferably the oil sump temperature is measured as the admission temperature, characterized a) in that the sensor device is designed to measure a second oil temperature downstream of the oil cooler and upstream of the internal combustion engine and to determine a temperature difference (ΔT) from the admission temperature minus the second oil temperature as the second parameter; and b) in that the evaluation device identifies the occurrence of a malfunction of the oil thermostat, b1) when the admission temperature lies below the first temperature threshold value, at least one predetermined reaction period has elapsed since the admission temperature has been below the first temperature threshold value, and the temperature difference (ΔT) is greater than a first threshold value (ΔT1); and/or b2) when the admission temperature is above the first temperature threshold value at least one predetermined reaction period has elapsed since the admission temperature has been above the first temperature threshold value, and the temperature difference (ΔT) is smaller than a second threshold value (ΔT2); and/or b3) when, after the first temperature threshold value is exceeded by the admission temperature, an increase of the temperature difference (ΔT) proceeds more slowly than a predetermined desired increase of the temperature difference (ΔT). b1) when the admission temperature lies below the first temperature threshold value, at least one predetermined reaction period has elapsed since the admission temperature has been below the first temperature threshold value, and the temperature difference (ΔT) is greater than a first threshold value (ΔT1); and/or b2) when the admission temperature is above the first temperature threshold value at least one predetermined reaction period has elapsed since the admission temperature has been above the first temperature threshold value, and the temperature difference (ΔT) is smaller than a second threshold value (ΔT2); and/or b3) when, after the first temperature threshold value is exceeded by the admission temperature, an increase of the temperature difference (ΔT) proceeds more slowly than a predetermined desired increase of the temperature difference (ΔT).