EPR spectra in the characteristic frequency range and H-1, C-13, and O-17 NMR spectra of fresh, used, and aged (under model conditions) GK grade transformer oil are recorded and analyzed. The EPR spectra show signals from Er3+, Yb3+, Fe2+, and Fe+ in the oxygen environment, and Ti3+ in the sulfur environment. It is shown that 1H NMR spectroscopy is a fast, informative, and reliable method of determination of moisture content of transformer oil. The data obtained confirmed high efficiency of EPR and NMR spectroscopy for analyzing transformer oil and monitoring its hydration processes.