In recent years, oil spill pollution resulting from maritime accidents has affected several points around the globe. After the sinking of the Prestige oil tanker off Galicia, this problem acquired high relevance in Portugal. The portuguese coast and its EEZ are located in an area of intense maritime traffic, and therefore are under the dangerous threat of oil pollution from maritime accidents. Studies of oil-spill modeling, both offshore and in estuarine areas, have been carried out but, until present, no research on this topic has been done for the Ria de Aveiro lagoon. This is relevant because this of the largest and busiest ports in the country is located in this barrier/lagoon system, which is also an environmentally protected area that is a habitat for many species. In this work we estimated the dispersion through Ria de Aveiro of an offshore oil-spill that reaches the Barrier/lagoon mouth. The approach used consists of the Lagrangean modeling of passive particle emissions. The Lagrangean model is coupled to a hydrodynamic model, previously calibrated and validated for this lagoon. The simulations were carried out for 3 different tide ranges (neap, spring and average) and took into account the average rivers freshwater input. In order to simulate cases of extreme drought, simulations without rivers freshwater input were also performed. The results show that the northern area of the Ria de Aveiro lagoon would be the most affected, especially the S. Jacinto and the Espinheiro channels, and that the oil-spill only takes a few hours to spread through theses channels if it reaches the lagoon during the flooding period.