In order to improve the understanding of the drive mechanism of the main jet deflection inside a fluidic oscillator, a 3D large eddy simulation and a visualization experiment are performed. The distribution of vortex cores, pressures, and mass flow rates through certain planes are discussed emphatically to explore the process of oscillation. The pressure difference inside the fluidic oscillator and the mass flow rate for some certain planes are compared. A kind of sine wave with certain phase differences is found for these two flow parameters. The results show that the vortex cores distributed between the wall of the chamber and the main jet and the pressure differences between the left and right sides would drive the deflection of the main jet. In addition, the max pressure difference is found to be located in the inlet of the feedback channels, which would also help to promote the process of deflection.