Transient thermal system management is promising in various engineering disciplines such as thermal management of electronics [CKK98] and aerospace systems [DYB17]. There is a need for an approach to estimate the system’s complex status to exert control and perform design optimization. Such an approach should have a low calculation cost while capturing the essential flow physics such as the phase change heat transfer and nucleate boiling. As a representative of transient thermal systems, Oscillating Heat Pipes (OHPs) consist of a serpentine capillary channel partially filled with liquid that is embedded in a thermally-conducting solid. The model reported here aims to capture the essential physics of an OHP with minimal complexity and treats some parameters typically derived from correlations or experiments (such as the film thickness and film triple point velocity) as functions with tunable constants to be estimated by data assimilation. This model contains two modules. The first uses a novel and flexible formulation of the conducting solid, solving the two-dimensional heat equation in a thin plate, with evaporators and condensers as immersed forcing terms and the OHP channel as an immersed line source. The second module solves one-dimensional fluid motion and heat transfer equations within the fluid-filled channels based on mass, momentum, and energy conservation, nucleate boiling, and bubble dryout. It extends the commonly-used film evaporation-condensation model, allowing both variable liquid film thickness and length and thereby enabling the model to capture dryout. These modules are weakly coupled, in that wall temperature in the channels is obtained from the first module and heat flux from the channels determines the line source strength. After minimal training, the thermal conductance calculated by this model shows good agreement with a wide range of experiments performed by Drolen et al. [DWT22]. In particular, the model successfully predicts the experimentally-observed transition from stable OHP operation to dryout, for the first time to the authors’ knowledge. Then the model is used to study the gravity effects of a ground-tested OHP and explored possible reasons of the discrepancy between the simulation and the ground experiments. This thesis also shows some data-assimilation techniques performed, such as using ensemble-Kalman Filter to estimate condenser contact condutance using a simple model adapted from Zhang et al. [ZFS02] and using Markov Chain Monte-Carlo method to sample inertial thickening parameters and plate thickness using the proposed model.