Synchrotron-based high-pressure and temperature single-crystal X-ray diffraction experiments were conducted on two hydrous orthoenstatite samples (oEn#1: Mg1.004Si0.996O3, 619ppm water; oEn#2: Mg0.947Ni0.055Si0.998O3, 696ppm water) to 34GPa and 700K, using resistively heated diamond anvil cells. The -opx (Pbca space group)-opx (P2(1)/c space group) phase transition of oEn#1 occurs at 12.90(2)GPa, and the -opx phase persists to 34.25(1)GPa. The - transition of oEn#2 occurs at 13.50(1)GPa, and a new isosymmetric -opx-opxII transition takes place at 29.80(4)GPa. The -opxII phase is preserved down to 24.53(3)GPa during decompression. The transition to the monoclinic -opxII phase is interpreted as a result of incorporation of Ni2+ into the orthoenstatite structure. Fitting the third-order Birch-Murnaghan thermal equation of state to the single-crystal P-V-T data yields the thermoelastic parameters of the - and -opx phases for both orthoenstatite samples. This study is the first attempt to determine the thermal equation of state of the -opx phase. Our results suggest that several hundred ppm of water has negligible effects on the bulk modulus of orthoenstatite but notably enhances the thermal expansion. The potential effects of metastable orthoenstatite on subduction zone dynamics are discussed, and the possible contributions of displacive phase transitions to enhancement of the transformational faulting mechanism of the deep-focus earthquakes in subducted slabs are considered. The presence of metastable orthoenstatite within cold slabs could promote slab stagnation above the 660-km discontinuity.