The self-excited oscillating pulsed jet apparatus has attracted considerable attention due to its simple structure, ability to generate passive oscillatory cavitation, and ease of engineering application. Nevertheless, the majority of extant studies on self-excited oscillating pulsed jets focus on homogeneous water-based media, while research on the flow and mixing characteristics of multiphase media remains relatively scarce. This limitation hinders its application in enhancing liquid-liquid heterogeneous phase mixing processes. To elucidate the mixing mechanism of oil-water heterogeneous media in the jet apparatus, this study employs oil as the continuous phase and water as the dispersed phase. Numerical simulations were conducted using the realizable k-ε turbulence model, Euler multiphase flow model, and population balance model (PBM), with simulation accuracy validated through experiments. Based on this, the mixing process of oil-water heterogeneous media inside the jet apparatus and the droplet size distribution characteristics of the dispersed phase were simulated and analyzed. The results indicated that oil-water heterogeneous media generated cavitation bubbles with size periodic variations and pulsating cavitation jets inside the jet apparatus. Under the synergistic action of turbulent inertial forces and viscous shear forces, dispersed-phase droplets were broken up, thereby promoting the mixing of the oil and water phases. Further analysis revealed significant differences in the primary regions where turbulent inertial forces and viscous shear forces acted within the jet apparatus. Turbulent inertial forces were mainly concentrated near the outlet and inlet of the resonator cavity, facilitating fluid mixing in this region, whereas viscous shear forces mainly produced near the collision wall of the resonator cavity and the lower nozzle channel wall, enhancing the mixing process in these areas. The combined effect of turbulent inertial forces and viscous shear forces led to the continuous breakup of dispersed-phase droplets, effectively intensifying the mixing process of oil-water heterogeneous media. In addition, mixing by jet apparatus can obtain narrowly-distributed dispersed-phase (water) droplets.