Fluctuations in offshore low-temperature heat sources and cold seawater alter the heat input, heat rejection, and condensing pressure of organic Rankine cycle (ORC) systems, making reliable off-design assessment important for marine power conversion. This study investigates a closed R134a ORC subjected to hot- and cold-source temperature and flow variations. A variable-condensingpressure model is developed in Modelica. The evaporator and condenser are discretized using the finite-volume method, and receiver pressure is determined from coupled mass and energy balances; the working-fluid mass flow is maintained at 0.50 kg/s. Steady-state matrices are used to quantify pressure, phase state, net power, and thermal efficiency. At 363.15 K and 2.50 kg/s on the hot side and 288.15 K and 1.53 kg/s on the cold side, the model gives high/low pressures of 2.496/0.793 MPa, net power of 4.557 kW, thermal efficiency of 4.66%, and receiver level of 0.501. Across the normal cold-source-temperature cases, condensing pressure and net power vary by 29.7 kPa and 0.185 kW, respectively; the corresponding ranges over the cold-source-flow matrix are 35.5 kPa and 0.224 kW. The 359.15 and 360.15 K hot-source cases lead to wet expansion, whereas the 278.15 and 283.15 K cold-source cases cross the pump-inlet phase boundary. Refined calculations over the 89-90 °C hot-source interval show increases of 0.128 kW in net power and 1.532 kW in evaporator heat input, with thermal efficiency increasing from 4.606% to 4.664%. These results quantify the distinct effects of heat-input and heat-rejection variations under variable condensing pressure and identify the associated phase boundaries.