The ultrasonic effect on the heat transfer performance in oscillating heat pipes (OHPs) was investigated experimentally. Ultrasonic sound was applied to the evaporating section of the OHP by using electrically controlled piezoelectric ceramics. The heat pipes were tested with or without the ultrasonic effect. The effects of heat input, filling ratio, orientation, operating temperature, and working fluids (water and acetone) were investigated. The experimental results showed that ultrasonic sound can affect the oscillating motions and enhance the heat transfer performance of an OHP. However, the heat transfer enhancement mainly occurs at low heat input. In addition, it was found that heat transfer enhancement of the ultrasonic effect depends on the working fluid and operating temperature. At an operating temperature of 20 °C, the enhancement percentage of the water OHP is higher than acetone OHP. However, when the operating temperature was increased to 40 °C, the enhancement percentage of the water OHP was lower than the acetone OHP.