Oscillating heat pipe (OHP) is highly efficient heat transfer devices with simple structures and versatile configurations, making them valuable for thermal management in complex environments. For equipment operating under variable load conditions, rapid startup is critical to ensuring reliability and operational safety. A heat-source-assisted approach was employed to experimentally investigate a three-dimensional copper-water OHP. The results indicate that while the auxiliary heat source stabilizes temperature fluctuations, it also suppresses the formation of unidirectional flow, leading to higher evaporator temperatures, increased thermal resistance, and reduced overall heat transfer performance. Nevertheless, the auxiliary heat source prevents stagnation during startup, allowing the OHP to bypass the conventional startup stage and achieve significantly faster activation. Based on these findings, a heat-source-assisted strategy was proposed: the auxiliary heat source was activated during the initial startup phase to quickly induce oscillation, and once stable operation was established, it was switched off. This strategy shortens the OHP startup time without compromising heat transfer performance, thereby enabling rapid response to sudden thermal loads. Under a 50 W input, the OHP's time to reach stable operation decreases from 33 s to 11 s, while at 150 W, it is reduced from 7 s to 5 s. The research results provide a novel and effective solution for efficient thermal management of OHPs under abrupt load variations.