<p>Out-of-step splitting protection is an important self-rescue means for power system. However, how to quickly search for the optimal out-of-step splitting section (OSS) satisfying multiple constraints in a very short time and implement splitting online is a major challenge, especially for the multi-oscillation mode. To this end, this paper carried out relevant research. Firstly, it analyzes the difficulties faced by the OSS search work under the multi-oscillation mode from the perspective of optimality and rapidity. Secondly, it studies how to quickly search the optimal OSS in the multi-oscillation mode. On the one hand, it presents an initial splitting section searching strategy suitable for the multi-oscillation mode. It quickly locates the initial OSS based on the voltage frequency of the system nodes and their connectivity and directly limits the search space around it. On the other hand, it gives a “Handshake” optimal OSS search strategy, with which the nodes on both sides of the initial OSS participate in the redistribution process, thereby minimizing the unbalanced power in each island after splitting and reducing the risk of cutting synchronous generators or loads. Therefore, the proposed method solves the contradiction between the rapidity and optimality of searching for OSS under multi-oscillation mode. Finally, experiments verify the proposed methods can quickly discriminate the appearance of the initial splitting section once it is formed in the multi-oscillation mode, and can online determine the optimal OSS with the smallest unbalanced power.</p>