Observing the transformer oil quality is essential for the transformer's health and system. The aging state of the transformer oils and the average remaining life accordingly can be determined using the breakdown voltage (BDV) test, ultraviolet-visible (UV-Vis) spectroscopy, and the refractive index (RI) standard methods. The subject of this study is to design and manufacture a much simpler and low-cost, evanescent field absorption-based fiber optic sensor (EFA-FOS) compatible with these proven standard methods and to perform a comparative performance analysis by determining a critical voltage threshold. Apart from the expensive and cumbersome standard methods mentioned above, the usability of the oil is carried out, thanks to the online sensor system, with the pretest using this critical voltage value. EFA-FOS measurement results of oil samples can determine the samples' aging degree with high accuracy. The samples' RI is determined with EFA-FOS with a sensitivity of -70 V/RIU and a linearity of 0.98 (R2). A strong relationship has also been determined between the sensor output and measured BDV with a regression constant of 0.86 (R2).