Magnesium oxide (MgO) is a prototypical strongly bonded ionic solid and an expected major constituent of rocky planet mantles and gas giant cores, yet its high-pressure phase diagram and shock response remain incompletely understood. Here we combine laser-driven decaying-shock and steady-shock compression with nanosecond X-ray di!raction, velocimetry, and optical pyrometry to investigate phase transitions in single-crystal MgO compressed to 600 GPa and 13,000 K. We observe a pronounced crystallographic orientation dependence in the B1 → B2 transformation pathway and subsequent melting behavior. For compression along [111], the shock-temperature evolution exhibits distinct plateaus consistent with equilibrium B1→B2→liquid transitions. In contrast, compression along [110] produces a single temperature discontinuity and di!raction signatures indicative of B2 nucleation from a transient disordered layer at the shock front. The combined thermodynamic and structural data indicate that near-instantaneous disordering can mediate stress relaxation under rapid uniaxial compression. Above 572 ± 24 GPa, all orientations converge to complete melting at 13,200 ± 600 K, constraining the B2–liquid boundary at multi-megabar pressures and identifying MgO as the highest-temperature solid measured at planetary interior conditions. These results demonstrate that crystallographic orientation governs phase-transition pathways in strongly compressed ionic solids and provide a benchmark for modeling matter under extreme planetary interior conditions.