Various embodiments of an oil monitoring apparatus are provided. In one embodiment, an oil monitoring apparatus includes a probe and an analyzing module in-line connected to the probe. The probe includes: a first sensor for measuring respective optical intensities of a light passing through the oil at respective red, green and blue wavelength ranges; a second sensor for measuring a water content; and a third sensor for measuring a temperature of the oil. The analyzing module calculates and monitors chemical deterioration of the oil, total contamination of the oil, a relative saturation of the oil by water and temperature of the oil based on the output signals of the first and third sensors. The oil monitoring apparatus monitors various parameters relating to the deterioration of the oil and to the physical properties of the oil simultaneously and successively.