Deflection barriers are often constructed to divert debris flows to less hazardous areas, where oblique shocks form upon impact. This study presents theoretical models for oblique shock against a deflection barrier, which predicts the runup height and impact load of oblique shocks with known incoming-flow conditions. Flume experiments are then conducted with the flume inclination and gate opening height as variables to quantify the incoming flow conditions via the Froude number, and to validate the performance of the oblique shock model. The experimental results show that both runup height and impact load (in dimensionless form) increase with the incoming-flow Froude number, which is well predicted by the theoretical models. The runup height and impact load of normal shocks can be considered as upper limits of oblique shocks. However, it would be overly conservative to adopt the predicted runup and impact load of normal shocks for oblique-shock mitigation. It should be noted that the proposed model is not applicable in scenarios where dead zones form between the deflected flows. Findings of this study lay a theoretical foundation for the design of mitigation structures aimed at diverting debris flows.