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This paper studies the channel exploration problem for the distributed opportunistic spectrum access (D-OSA) systems, where multiple secondary users (SUs) sequentially sense multiple licensed channels and access one of idle channels. SUs operate on a frame-by-frame basis and need to sense channels at the beginning of each frame. According to the stopping rule, SUs determine to stop at the sensed channel or continue to sense next channels. However, the optimal design in the scenario of multiple SUs with the multiple licensed channels is difficult to realize for D-OSA systems. To solve this problem, we introduce an adjusted function considering the interactions among SUs, and then propose an adaptive stopping rule to improve the performance of the multi-user system. Simulation results are provided to show the effectiveness of the adaptive stopping rule and the significant improvement of the throughput performance.