While cation order-disorder transitions have been extensively investigated because of their decisive impact on chemical and physical properties, only few anion order-disorder transitions are known. Here, we show that Sr2CuO2Cl2-type layered perovskite Sr2LiHOCl2 exhibits a pressure-induced H-/O2- order-disorder transition. When synthesized at ambient and low pressures (≤2 GPa), Sr2LiHOCl2 is isostructural to orthorhombic Eu2LiHOCl2 (Cmcm) with a H-/O2- order at the equatorial sites. However, applying a higher pressure (5 GPa) during synthesis causes the equatorial anions to be disordered, leading to a tetragonal symmetry (I4/mmm) with a loss of the superstructure. The structural analysis revealed that, in the ambient pressure phase, HLi2Sr4 and OLi2Sr4 octahedra have distinct sizes to stabilize otherwise underbonded oxide ions, which is less important at the higher pressure. Anion-disordered Sr2LiHOBr2 and Ba2LiHOCl2 were also obtained at 5 GPa. Given the abundant layer-type anion order in perovskite-based oxyhydrides (e.g., La2LiHO3), the inclusion of additional anions (e.g., chloride) expands the frontiers of anion ordering patterns and their distribution control with the benefit of improving ionic conduction in solids.