An optical sensor for transformer oil monitoring is reported. It has been implemented in a plastic optical fiber which acts as optical platform for the sensing element, that is, a Molecular Imprinted Polymer layer, which is a robust, reproducible and low-price receptor for chemical marker in the insulating oil matrix. This optical sensor has been assessed on the detection of a chemical byproduct of transformers’ paper degradation, the furfuraldehyde (2-FAL). The optical sensor performances have been tested on used insulating oil samples spilled from ex-serviced instrument transformers and compared with results of standard methods applied to the same samples. The achieved good agreement emerging by comparative analysis confirms the ability of the optical sensor of detecting low 2-FAL contents directly in mineral oil, without any treatment or dilution of the oil.