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The wind-turbine step-up transformers are subjected to high-frequency high dV/dt transient voltages due to the operation of power electronic converters and vacuum circuit breakers. These high-frequency transients are harmful to the transformer turn-to-turn and layer-to-layer paper-oil insulation. The current research work addresses the effect of the transient voltage parameters on the degradation of the transformer paper-oil insulation. Two-level, two-factor design of experiments were used to obtain the effect of transient voltage parameters on the aging of the paper-oil insulation. Rise time and switching frequency were the two factors (parameters) selected for the aging experiments. The two values of rise time were 220 and 650 ns, and two values of the switching frequency were 1 and 3 kHz. The faster rise time (220 ns) and higher switching frequency (3 kHz) of the voltage led to a higher degradation for the paper-oil insulation. In addition, the transient voltage aging of paper-oil insulation is investigated and compared for two types of mineral oils: Luminol TRi oil and Voltesso 35 oil, which have differences in their electrical and chemical properties.