Intraseasonal oscillation (ISO) is a dominant mode of natural variability during the South China Sea (SCS) summer monsoon (SCSSM) and plays a crucial role in modulating extreme events, such as droughts and floods. Therefore, studying the long-term changes in ISOs during the SCSSM is essential for understanding regional rainfall variability and extremes. Based on the observational and reanalysis datasets from 1980 to 2019, this study identifies a significant increasing trend in ISOs over the central SCS, accompanied by a rise in its contribution to total monsoon variability at a rate of ~3% per decade. A corresponding increase in intraseasonal moisture variability is also observed, consistent with the key role of moisture in maintaining ISO deep convection. The intraseasonal moisture budget analysis demonstrates that vertical moisture advection is the primary dynamic process responsible for the intraseasonal moistening in the core region, in particular the advection of background moisture by intraseasonal vertical motion. In addition, this advection enhancement is largely attributable to a steepened background moisture gradient, which results from increased low‐level moisture. Moreover, the low-level moisture enhancement is driven by the enhanced surface latent heat flux, ultimately linked to oceanic warming in the central SCS. These findings present the multi-timescale air-sea interaction, whereby the oceanic thermal forcing modifies ISOs during the SCSSM. These results provide insights for understanding the regional climate responses to ocean warming, and for improving monsoon predictions and projections as well as early warnings of extreme events.