This work presents the results obtained in the measurement of complex dielectric permittivity of some samples of oil from high voltage transformers within a frequency range comprised between 100 MHz and 3 GHz. Three types of oil samples were considered: new, used and those used samples that have undergone a filtering process for their recovery and new use. The measurements were performed by using a low-cost probe adapted from an open-ended SMA coaxial connector. The values obtained in a vector network analyzer of the reflection coefficient, measured on the contact surface between the probe and the oil are used to calculate the real and imaginary part of the complex dielectric permittivity by using theoretical models of the cable's open end. The results obtained permit assigning trust to this measurement and characterization method of the oils according to the value of the complex dielectric permittivity, given that it agrees with some values reported in the literature obtained through different methods.