The maintenance of the transformer is more and more important and the analysis of moisture content in transformer oil has become a very important part of transformer maintenance. Water is known as a strong absorber for THz radiation due to its distinctive intermolecular vibration in this frequency range. In the present work, we analyzed transformer oils regarding their water content by THz spectroscopy. Based on THz time-domain spectroscopy, information about the absorption coefficient and the refractive index of the oil samples is provided. By comparing the results with relevant theories we demonstrate that both the absorption coefficient and the refractive index are linearly related to the concentration of water in the oil solutions.