Transformers are crucial elements in the transmission and distribution of electrical energy. The importance of diagnosing these equipments are two-fold: (1) the necessity of service reliability and (2) the likelihood to avoid economic and environmental concerns. Under service conditions, the electrical and thermal stresses or chemical contaminants may degrade the insulation oil inside the transformer and cause incipient failures or reduce its service life. Partial discharges well recognized to be among the most common stresses that can lead to slow but steady degradation of insulating oil in transformers. The present work aims at understanding the influence of low energy electrical discharge on mineral oil based on two spectroscopic methods: FTIR spectroscopy and Frequency Domain Spectroscopy (FDS). An electrical fault has been created by continuous discharge of 10 kV on the surface of various oil samples according to the ASTM D6180. From the FDS results, it was found that the amount of charge carriers and moisture increased with the aging time elapsed that influences the conduction phenomena and in turn, increases the dissipation factor. These results are confirmed by the FTIR results, which show that the intensity of the peak absorbance of the C-H and C-C functional group decreased with aging. The application of these two methods may help monitoring the condition of oil. A combined FTIR and FDS measurements highlighted the correlations between modifications in electrical properties and changes in the chemical structure of the oil under electrical accelerated ageing.