ABSTRACT: Oil spills caused by ship collisions and offshore oil wells are an ongoing risk for estuaries in the northern Gulf of Mexico. The fate and transport of the oil spill across the interface between a bar-built estuary and the adjacent coast are influenced by multi-scale forcing mechanisms and their corresponding interactions. Of primary interest are the alongshore currents on the shelf encountering strong tidal flows at the estuary entrance. A new cross- scale model was developed for Galveston Bay to reproduce the multi-scale flows. The model was employed in regionally-distributed numerical Lagrangian experiments to investigate the oil spill transport across the shelf- estuary interface. The influence of the multi-scale flows on the oil spill transport was characterized in terms of Lagrangian connectivity and Lagrangian flushing. The new Galveston Bay model was also used to evaluate the Texas City “Y” spill and resulted in a reasonable agreement with the NOAA observations. This research enhances our understanding of the oil transport across the threshold between two contiguous water systems and highlights the importance of resolving the multi-scale flows for the purpose of oil spill predictions.