Abstract: Dynamic imbibition, which is significantly affected by flow rate, plays an important role in the development of tight oil. This study investigated the impact of flow rate on dynamic imbibition in fractured tight sandstone cores via online nuclear magnetic resonance core-flooding experiments. The oil expulsion efficiency and capillary number of multiscale pores were quantitatively analyzed to elucidate the influence of flow rate on the oil recovery during dynamic imbibition. The pores of the cores used were divided into micropores (0.01–1.00 μm in diameter), mesopores (1.00–30.00 μm in diameter), and macropores (30.00–400.00 μm in diameter) by matching the T2 spectrum and the mercury intrusion data. The volume proportion of micropores was 52.0%, and that of macropores was 19.0%. The total oil recovery of the core was found to reach 29.8% at the optimal flow rate of 0.1 mL/min. At the optimal flow rate, the oil recovery of micropores reached 50.4%, followed by that of macropores (28.6%), and that of mesopores was the lowest (15.8%). The oil expulsion efficiency, the capillary number, and the contribution to total oil recovery of micropores significantly increased with the decrease in flow rate, while those of macropores decreased. This was caused by the synergy of capillary force and displacement pressure. During dynamic imbibition at a low flow rate, the oil in micropores was effectively expelled driven by capillary force, and the effect of displacement pressure was weak, leading to large amounts of remaining oil trapped in macropores. On the contrary, when the flow rate was too high, large amounts of remaining oil would be trapped in micropores. Only at a moderate flow rate did the capillary force and displacement pressure both have significant effects on oil expulsion, and the oil in different sized pores was effectively expelled, thus generating a relatively high total oil recovery.