This article develops a concept for a suitable startup condition for a closed-loop oscillating heat pipe (CLOHP). This concept was developed by using visual data and the thermodynamics theory for predicting the amount of vapor evaporation and condensation in a CLOHP. The visual data indicated that the key to a suitable startup is the amount of net vapor expansion in the evaporator and the amount of net collapsed vapor in the condenser. Initial dryout, an event that occurs after a startup failure, results when the net vapor expansion is higher than the amount of net vapor collapsed. This situation obstructs the replacement process. This is a mechanism in which the volume of mixture from the condenser section flows to the evaporator section to replace the volume of mixture that leaves the evaporator section. When the replacement process is impeded, all of the liquid in the evaporator section evaporates and the evaporator section is not refilled by the mixture from the condenser section. The evaporator section is then filled with vapor and initial dryout occurs. In addition, this article presents a mathematical model that predicts the operating temperature for a suitable startup condition. This prediction can be used to avoid a startup failure of a CLOHP. When comparing the model with that of the experimental data, a 16% error range was attained.