Oceanic slab vertical tearing is prevalently identified in the present Earth. More general background for vertical slab tearing is the transform-fault subduction during horizontally-oblique tectonic convergence. However, its geodynamic mechanisms are poorly understood to date. This work introduces a full numerical 3-D time-dependent Stokes’ thermo-mechanical flow model to investigate the characteristics and mechanism of vertical tearing of active transform-faulted oceanic slab during oblique subduction. We find that (i) transform-fault ages-offset and (ii) subduction horizontal obliqueness have the first-order control, even without the lateral physical-property differences. The overriding plate enforces (surface contact interaction) bending of one slab first, which superimposes the differential sinking driven by slabs-age-thickness differences. For obliqueness angles ≥30° and/or age-ratios of the secondly-bent to the firstly-bent slab being <0.6, well-developed slab vertical tearing is unavoidable inside the mantle. Quantifying the horizontal distance vector between sinking slabs, we find that young overall lithosphere (average <30 Myr, for any age ratio) at high subduction obliqueness angles (>~25°) tends to produce trench-parallel slab tearing. In contrast, combinations of small-intermediate obliqueness angles (0-30°) and age ratios with the slab that bends at the trench first being relatively older-thicker, tend to produce trench-perpendicular tearing, which is related to differential slabs-hinge retreat or rollback. These numerically-predicted scenarios and phenomena are consistent with plate-tear imaging results from subduction zones. Our modeling results also suggest that the continual along-trench variation in subduction dip angle may be related to oblique subduction’s early stages of evolution.