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Generally, the oscillating heat pipe (OHP) diameter should be kept below a specific critical size. As a result, the working fluid forms a vapor-liquid slug train under the capillary action, producing a gas-spring-mass mechanical vibration system. Determining the critical diameter is essential for the OHP to form this slug train. A conventional critical diameter model only considers the surface tension and gravity. However, the vapor bubble flow in the liquid slug plays a key role in forming this essential slug train. Even if the hydraulic diameter of an OHP is much larger than the conventional critical diameter, the OHP can still function. The dynamic flow significantly affects the formation of the vapor-liquid slug-train. Therefore, a semi-empirical model was developed considering the effects of the dynamic flow and the heat input, filling ratio, and working fluid. The concept of dynamic critical diameter (DCD) was proposed. The model shows that the OHP DCD increases when the heat input increases for a given evaporator temperature. In addition, the model can predict that the OHP DCD decreases when the evaporator temperature increases. • The concept of dynamic critical diameter (DCD) was proposed. • The mechanism of vapor-liquid plugs formation in OHP with pipe diameters exceeding conventional MCD is revealed. • A semi-empirical DCD model was developed. • The filling ratio should be between the values of 0.24 and 0.75 when the diameter exceeds MCD. [ABSTRACT FROM AUTHOR]