<p>In power equipment utilizing oil–paper insulation, the overlap structure is particularly susceptible to discharge due to its inherent structural characteristics, compared to regions with uniform dielectric distribution. This study developed an experimental platform to investigate the effects of AC and AC-DC composite voltages on oil–paper insulation overlap structures. Breakdown tests were conducted under three conditions: room temperature, high temperature, and high moisture, with a detailed analysis of the electric field. The results demonstrate that increasing the overlap distance enhances electric field uniformity across all conditions, leading to higher discharge voltages, especially under AC-DC composite voltages. The introduction of a DC component reduces the maximum electric field strength experienced by the samples, resulting in increased discharge voltage under AC-DC conditions. Conversely, elevated moisture raises the conductivity of the samples, alters the charge density at the interface, and diminishes insulation strength. Notably, the benefits of increasing overlap distance are less pronounced under high moisture conditions. Therefore, it is advisable to minimize moisture levels in oil–paper insulation overlap structures in engineering applications to enhance performance and reliability. These findings suggest that optimizing overlap distances and controlling moisture levels can enhance the reliability of oil–paper insulation in transformers, particularly under composite voltage conditions.</p>