The elastic properties of synthetic low-pressure and high-pressure CaAlSiO oxygen defect perovskites were investigated by in situ X-ray diffraction in a large-volume high-pressure apparatus. The P- V- T data were collected up to 22.75 GPa at room temperature, up to 12.88 GPa and 1,300 K for low-pressure phase and up to 25.76 GPa at room temperature for high-pressure phase. The P- V data at room temperature were fitted using a third-order Birch-Murnaghan equation of state to obtain K = 146.1(9) GPa and $$K_{0}^{\prime }$$ = 3.64(9) for the monoclinic low-pressure phase, and K = 150.4(19) GPa and $$K_{0}^{\prime }$$ = 3.16(23) for the rhombohedral high-pressure phase. If $$K_{0}^{\prime }$$ was fixed at 4.0, the isothermal bulk moduli were obtained as 142.6(3) and 144.0(8) GPa for low-pressure and high-pressure phase, respectively. Both the low-pressure and high-pressure CaAlSiO oxygen defect perovskites are much softer than pure CaSiO perovskite. The P- V- T data of low-pressure phase were fitted by the high-temperature Birch-Murnaghan equation of state to get thermoelastic properties as V = 827.0(3) Å, K = 146.8(22) GPa, $$K_{T}^{\prime }$$ = 3.55(21), (∂ K/∂ T) = −0.037(2) GPa/K and α = 7.67(20) × 10− 3.20(30) × 10 T. Based on the results, the density profiles of low-pressure and high-pressure CaAlSiO oxygen defect perovskites were calculated and compared with those of some mantle silicate minerals to discuss the potential occurrence of CaAlSiO oxygen defect perovskite in the Earth's interior. [ABSTRACT FROM AUTHOR]