The offshore S oilfield has entered a high water cut development stage, yet a significant amount of residual oil remains, indicating potential for further enhanced oil recovery. This study evaluates the ultimate oil displacement efficiency of the S oilfield under typical permeability and crude oil viscosity conditions through high multiple waterflooding experiments, thus clarifying the EOR potential during this period. Using online nuclear magnetic resonance testing, the distribution pattern of residual oil during the high water cut stage is revealed. Additionally, by conducting high multiple waterflooding and online nuclear magnetic resonance displacement experiments, the study investigates the ultimate oil displacement efficiency and the mobilization pattern of residual oil in the offshore S oilfield after waterflooding, thereby determining the EOR potential with binary flooding post-waterflooding. The research indicates significant potential for enhancing oil recovery after water flooding in the offshore S oilfield. The ultimate oil displacement efficiency with high multiple water flooding reaches up to 75.2%, which is 17.6 to 25.8 percentage points higher compared to the high water cut stage. During the high water cut stage, residual oil is mainly distributed in pores with radius >10 μm and 1~10 μm, accounting for 32.4%~55.1% and 35.1%~47.7%, respectively. After injecting the binary system, the water cut decreases, and oil production increases significantly, with the ultimate oil displacement efficiency rising to 84.8%, an increase of 33 percentage points, requiring less than 2 PV of subsequent water flooding. The binary system effectively mobilizes residual oil in pores of different sizes by increasing sweep volume and oil displacement efficiency. Therefore, binary flooding after water flooding in the offshore S oilfield can effectively enhance oil recovery and significantly reduce development time costs.