This study investigates the effect of gravity on the flow pattern and thermal efficiency of a single-loop oscillatory heat pipe. To simulate the influence of gravity, the deployment angles of the mechanism are varied (30°, 45°, 60°, and 90°). OpenFoam software is implemented to model boiling and condensation in the oscillating heat pipe, utilizing the volume of fluid (VOF) method. The evaporator is supplied with 55.5 W of heat power, the condenser wall temperature is maintained at 300 K, and the filling ratio of heat transfer fluid (water) is 40%. The findings revealed that decrease in gravitational force results in the thermal resistance be increased and the thermal performance of heat pipes be diminished. Expectedly, the best thermal performance in the oscillating heat pipe is observed in vertical mode, however, this study also examines the influence of reduced gravity. The simulation results show that the bubble pattern is first initiated by the bubble nucleation at the start of the heating process. Consequently, by bubble coalescence the slug and annular regimes can be observed. The phenomenological analysis of the dissolution, bubble coalescence, growth, and contraction observed in this study are discussed.