Oil-immersed transformers are widely used in power systems but face internal overheating due to overloading and extreme ambient temperatures. Heat dissipation primarily relies on natural convection of insulating oil, which is insufficient under extreme conditions. This study presents a numerical model of convective heat transfer in oil-immersed transformers using the finite element method. It examines how the gap between windings and transformer walls affects oil and winding hotspot temperatures under extreme conditions (40 °C ambient temperature, 1.2 times overload current). The results show that temperature reduction is most significant when the iron core is positioned at α = 0.13. Incorporating heat pipes improves heat dissipation, reducing hotspot temperature by 0.9 K and average temperature by 1.3 K. The temperature difference between low and high voltage windings decreases to 0.8 K with heat pipes. These findings provide insights for designing more efficient heat dissipation systems for oil-immersed transformers, especially under extreme conditions.