Unforeseeable growth in the global population needs growing technological advances in establishing power plants to overcome this critical issue by reducing greenhouse gases. In the present study, a thermodynamic analysis for a 626 MW supercritical oil-fired steam power plant under different operating loads of 50%, 75%, 100%, and 105% at a sliding-pressure operation is conducted. Based on the actual operation of the examined plant, the results show that the condenser has the highest energy loss. As well, at a 100% full load, 88.6% of the total exergy is destructed in the once-through steam generator, followed by the turbine, and then the condenser. Hence, a significant concern is introduced toward the steam generator since it has the largest exergy destruction percentage relative to other cycle components. The heat transfer sets inside a once-through steam generator are studied and analyzed. The exergy destruction in the combustion process represents 58.6% and 54% of the overall boiler exergy destruction at an operating load of 50% and 100%, respectively. In addition, the evaporator has higher exergy destruction in comparison with other heating surfaces in the furnace.