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Transformer oil is widely used in high‐voltage transformers as a cooling and insulation medium. Electrical and thermal stresses expose transformer oil to degradation, which may produce dissolved decay products (DDPs) that are partially adsorbed on the insulation paper and accelerate the degradation of oil and paper. An evanescent field D‐shaped plastic optical fibre (POF) sensor was fabricated using the side‐polishing technique to detect DDPs in transformer oil. A laser light source and power meter were used in the prototype to measure the output power of the POF sensor. The measurement of DDPs was conducted on seven samples of insulating oil collected from the field. The sensor performance was evaluated based on different sensing areas, and the sensor sensitivity was measured. Results show an excellent correlation between the output power and DDPs. A correlation analysis was conducted amongst sensor output power, transformer oil refractive index and area under the absorbance curve. The performance of the sensor demonstrates improved linearity and sensitivity of 97.49% and 26.6779 µW/RIU, respectively. The POF sensor is easily fabricated, low cost and can be used for real‐time monitoring of high‐voltage transformers. [ABSTRACT FROM AUTHOR]