It is shown that in the mineral transformer oil oxidation process the fluorescence spectrum of the oil shifts to the long-wave region. This phenomenon lies at the foundation of the method of determination of the degree of electroinsulating oil oxidation in real time. The oxidation state is determined by measuring the diagnostic parameter, i.e., the oxidation index that characterizes the magnitude of shift of the fluorescence spectrum of the oil upon its oxidation. The oxidation index is defined as the ratio of the fluorescence intensity measured in the longer-wave spectral range to the intensity measured in the shorter-wave spectral range. The results of measurement of the transformer oil oxidation degree by the fluorescence method vis-A -vis other measurement methods, such as titration and IR spectroscopy, are presented. The proposed method can be used to determine the increase in oil oxidation rate at the earlier stage. The device for implementing the developed method can be a portable one or can be installed in the oil-filled system of the equipment.