A detector for oil condition monitoring includes an optical fiber having a first end and a second end having an end face. A sensor body has a gap in which a sample of the oil may be received and a reflecting surface, the second end of the optical fiber being embedded in the sensor body and having an end face spaced from the reflecting surface across the gap. Light emitted from the optical fiber can pass through the sample of oil and be reflected by the reflecting surface back into the optical fiber. By interferometry of the respective signals, the condition of the oil can be determined.