The fuel-cooled oil cooler (FCOC) in the lubrication circuit plays a critical role in the aero gas-turbine engine's aerothermal management. However, the low temperature of the operating environment can congeal the oil and reduce the FCOC efficiency. The oil bypass valve (OBV) installed on the FCOC prevents pressure loss. Its failure may cause overheating, requiring preemptive performance prediction. Experimental and numerical analyses were used to evaluate the cooler's de-congealing performance under typical boundary conditions of pressure and temperature, OBV configurations, and re-routing of feed oil and fuel flow paths. The temporal variation of oil and fuel mass flow rates, temperature, and pressure of the feed oil and fuel provided an insight into the de-congealing process and duration. The experimental data were used to develop a one-dimensional (1D) flow and thermal network analysis model based on the effectiveness (e)-NTU method to predict the transient oil de-congealing performance of the FCOC. The customized commercial code predicted the de-congealing phenomena using empirical correlations with property correction schemes, showing good agreement with the experiment. The findings revealed various ways to enhance the de-congealing performance of the FCOC. The study results showed that the operating boundary conditions, OBV location and status, and flow arrangements affect de-congealing behavior and time. The present numerical model provides results quickly and can effectively predict experimentally costly and complicated cases. The attempted estimates of steady heat rejection and detailed methodology could guide future studies and practical applications.