The Oslofjord ecosystem experiences significant degradation due to cumulative anthropogenic pressures, including nutrient-induced eutrophication, overfishing, and habitat destruction. Existing institutional arrangements for coastal management in Norway have proven insufficient in addressing these complex challenges. In response, the Norwegian government launched an action plan specifically for the Oslofjord in 2021, aiming to restore a clean and healthy fjord that is accessible to all. This unique policy initiative can be studied through the lens of institutional layering. Rather than directly replacing existing institutions, layering describes the addition of new elements. Over time, the new layer may gradually shift the trajectory and influence of established institutions on societal behaviour and lead to transformative changes in policy outcomes. The design of the Oslofjord Plan, intended to “complement, coordinate, and reinforce” existing arrangements, can be understood as a deliberate attempt at layering, through the addition of instruments, actors, and changed perceptions. However, despite the Plan’s ongoing implementation, the ecological status of the fjord remains degraded. Our analysis offers two key explanations. First, several measures introduced by the Plan need time after being put into action before they start to work, underscoring the need to consider the temporal dimension when evaluating the Plan’s capacity to meet its objectives. Second, although the measures are beginning to shift institutional trajectories and societal behaviours, we argue that they remain insufficient to create transformative change. This is primarily due to institutional barriers embedded within existing institutional arrangements, which the Plan does not adequately address. These create lock-ins, constraining the Plan’s transformative potential.