Although optical circuit switch (OCS) based data center network (DCN) can offer high-bandwidth and low-power consumption interconnection for server blocks, it has slow reconfiguration speed and thus is not well-suited to handling highly bursty traffic. Despite several Valiant load-balancing (VLB) type algorithms designed to mitigate this mismatch, they cause notable residual traffic fluctuation, measured by second moment of traffic rate. This leads to a notable tail distribution of flow completion time (FCT). Thereby, this paper proposes a second-moment VLB (sVLB), leveraging the fact that all optical links from the same source rarely burst simultaneously. Different from previous VLB-type algorithms, sVLB utilizes spare capacity in the DCN for load balancing. Specifically, it offloads the bursty traffic from optical links to other intermediate blocks, employing only spare capacity on temporarily idle optical links, and these blocks subsequently route the bursty traffic to its destination. We introduce an sVLB-based framework, which additionally provides a suitable optical topology for sVLB to work with and a weight-based scheme to assist sVLB in intermediate block selection during bursty traffic offloading. Our qualitative assessment and extensive simulation demonstrate that the sVLB-based framework can smooth out the residual traffic fluctuation, thus outperforming existing strategies in normalized FCT.