<p>The onset of the summer monsoon marks an abrupt transition in the local annual hydrological cycle. As the earliest-established monsoon subsystem, the monsoon onset over the Bay of Bengal (BOB) not only influences early summer precipitation in the region but also plays a pivotal role in the subsequent progression of the Asian summer monsoon. Composite analysis in this study reveals that both the timing and northward propagation speed of monsoon-triggering intraseasonal oscillations (MTISOs) affect the reversal date of the upper-tropospheric meridional temperature gradient. Our results indicate that the occurrence of tropical deep convection is closely linked to Ocean–Land–Atmosphere interactions over the Indian Ocean, particularly modulated by the cross-equatorial flows (CEFs) over the BOB and the Somali region. Prior to monsoon onset, the springtime land sea thermal contrast (LSTC) acts as a key driver, influencing the strength of the BOB-CEF and consequently the initiation of MTISO. When LSTC is anomalously stronger, the BOB-CEF can establish earlier and then promote the development of tropical convection. In contrast, when LSTC is weaker, initiation of the MTISO is dominated by the Somali CEF. On a larger scale, diabatic-heating anomalies associated with Maritime Continent convection induce upper-tropospheric anticyclonic anomalies and further modify the vertical easterly shear over Southeast Asia, which can either promote or hinder the propagation of MTISO. In summary, anomalous monsoon onsets over the BOB are governed by a coupling process between lower-level Ocean–Land–Atmosphere interactions and upper-level circulation anomalies linked to Maritime Continent convection. This highlights the critical role of multi-scale interactions during the transitional season of the Asian summer monsoon.</p>