Rocks deep inside large planets experience pressures and temperatures far beyond anything on Earth. To find out how such rocks behave, we used first-principle computer simulations to compress and heat magnesium oxide (MgO)—the most abundant oxide component—to pressures of one gigabar and temperatures of 20,000 K. Our results show that the mineral’s familiar B1 crystal structure, stable throughout Earth’s mantle, gives way to the denser B2 form at about 600 GPa, a pressure comparable to that at Uranus’s center. At still higher pressures the B2 lattice becomes mechanically unstable, and we predict two phases, tetragonal P4/mmm and orthorhombic Pmma, that remain stable up to at least 1 Gbar. These phases feature unusually close like-ion contacts and a collapse of the electronic band gap, indicating a transition from insulating to metallic behavior. By tracking the equations of state and elastic properties across these transformations, we provide comparisons for extrapolation schemes commonly used in planetary modelling and can further inform models on how large planets cool and respond to tidal forces.