The filling ratio significantly affects oscillating motion and heat transfer in an oscillating heat pipe (OHP). When heat is added to the evaporating section, temperature and pressure increase. The pressure wave travels through both vapor and liquid phases. In the current investigation, focusing on the difference in pressure wave speed in the liquid phase compared to the pressure wave speed in the vapor phase led to the development of a mathematical model that predicts the filling ratio effect on the start-up power of a one-turn OHP. Results show that the heat input needed to start the oscillating motion in an OHP depends on the filling ratio. When the filling ratio increases, the heat input required to start up the oscillating motion increases; furthermore, there exists an upper limit. This upper limit of the filling ratio is dependent on the properties of the working fluid. (C) 2014 Elsevier Ltd. All rights reserved.