Magnesium oxychloride (MOC) cement is gaining renewed attention as a low-carbon construction material. However, conventional MgO production is constrained by the geographically limited availability of MgCO3 and its energy-intensive calcination at temperatures up to 1000 °C. Thermal recycling of waste MOC has so far been demonstrated in the 600–1000 °C range, leaving the lower temperature limits underexplored. This study investigates medium-temperature MOC recycling at 400–700 °C and assesses its feasibility by fully replacing virgin MgO with recycled magnesia powder (RMP). Results show incomplete decomposition and residual chlorides below 500 °C, the highest reactivity at 550–600 °C, where the active MgOα content exceeded that of the precursor, and a sharp decline in reactivity above 600 °C. Rehydrated MOC (R_MOC) retained up to 90% of the 28-day compressive strength of the reference mixture. Residual chlorides promoted Chlorartinite (Mg₂(CO₃)Cl(OH)·3H₂O) formation and favored Phase 3 (3Mg(OH)2·MgCl2·8H2O) over Phase 5 (5Mg(OH)2·MgCl2·8H2O). These findings define the lower temperature bound for energy-efficient MOC recycling and support the fully circular use of MOC waste.