•Mechanism of OSV in subcooled flow boiling was investigated theoretically.•OSV was supposed to be triggered by global bubble coalescence.•OSV resembles the transition from babbly to slug flows in two-phase flow.•Heat flux spent for vaporization was estimated using Chen's correlation.•Calculated OSV condition agreed with Saha-Zuber correlation well. Theoretical investigation was carried out for a possible mechanism of the onset of significant void (OSV) or net vapor generation (NVG) in subcooled flow boiling. Based on available knowledge on the two-phase flow regime transition from bubbly to slug flows, it was postulated that bubble coalescence is intensified substantially when the local void fraction near the heated wall exceeds a critical value. It was discussed that this can be a main cause of a rapid increase in the cross-sectional area-averaged void faction since the condensation rate decreases suddenly due to a sudden decrease in the interfacial area of vapor bubbles. It was demonstrated that if the vaporization rate on the heat transfer surface is evaluated using a reliable wall heat transfer correlation, the OSV conditions calculated by the present model agree with those by a widely-used empirical correlation well.