This article proposes a novel method for predicting frequency-domain spectroscopy (FDS) characteristics of oil–paper insulation at a temperature, which is different from the measurement temperature, through estimating activation energy. The predicted FDS characteristics curves at different temperatures show a very good agreement with each other when compared to the experimentally measured characteristics. From the FDS characteristics, two moisture-sensitive parameters are proposed for accurate estimation of moisture content of oil–paper insulation. The proposed technique is experimentally validated on several test samples with different moisture contents and also on a real-life transformer.