Compared with single-component systems, high-entropy boride (HEB) coatings exhibit superior oxidation resistance and ablation performance, yet the role of the high-entropy solid solution remains unclear. In this study, (Ti₁/₄Zr₁/₄Hf₁/₄Ta₁/₄)B₂ HEB coatings and mixed single-component boride (MIX) coatings were compared using air plasma ablation experiments and first-principles calculations to reveal the initial oxidation and product evolution. HEB coatings show slightly lower oxidative weight gain but a nearly 50% lower linear ablation rate than MIX coatings. Their oxidation products are dense, continuous multicomponent oxide solid solutions, whereas MIX coatings form mixtures of discrete single-component oxides. Calculations indicate that oxygen adsorption is slightly inhibited in the high-entropy system and that oxidation proceeds sequentially among constituent elements before forming multicomponent oxide solid solutions at high temperature. These dense oxide layers possess enhanced structural continuity and resistance to gas-flow erosion, accounting for the improved ablation performance. The results demonstrate that the high-entropy solid-solution structure facilitates the formation of stable protective oxide layers and thereby improves coating performance in ultrahigh-temperature environments. This study highlights the crucial role of high-entropy solid solutions in enhancing ultrahigh-temperature boride coating performance and offers guidance for their design and application.