Efficient thermal management is crucial in high-power electronic systems and renewable energy applications, where passive cooling technologies offer solutions that do not rely on external power sources. The helical oscillating heat pipe (HOHP) is a promising technology that combines high heat transfer capability with installation flexibility. Despite its potential, the thermal performance of HOHP is significantly influenced by the working fluid filling ratio and inclination angle. However, studies exploring the interaction between these parameters are limited. This experimental study investigates the effects of filling ratios (30%, 50%, 70%, and 80%) and inclination angles (80° and 90°) on the thermal performance of a 400-mm-long copper HOHP under varying heat loads. The temperature distribution and thermal resistance were measured using a calibrated thermocouple system and real-time data acquisition. The results indicate that a 50% filling ratio combined with a 90° inclination angle yields the most optimal thermal performance, achieving the lowest thermal resistance of 0.0419 °C/W at a heat load of 315 W. Conversely, an excessively high filling ratio (80%) reduces condensation efficiency due to increased fluid oscillation resistance. This study provides valuable insights for optimizing HOHP design and contributes to the development of reliable passive cooling systems for various industrial applications. © Published under licence by IOP Publishing Ltd.