Offshore wind electricity generation is prospected to increase substantially in the near future at a number of locations, like in the North and Baltic Seas, and emerge at several others. The global growth of offshore wind technology is likely to be accompanied by reductions in wind park construction costs, both as a result of scaling and learning effects. Since 2005, however, significant cost increases have been observed. The recent surge in commodity prices is one of the main drivers of these cost increases. We demonstrate that if one abstracts from material price fluctuations, in particular for metals such as copper and steel, turbine production plus installation cost data publicly available for a series of offshore wind park projects (realized in several European countries since the 1990s) show a cost reduction trend. Hence various other sources of cost increases, such as due to the progressively larger distances from the shore (and correspondingly greater depths at sea) at which wind parks have been (and will be) built, are outshadowed by cost reduction effects. Expressing the overall cost development for offshore wind energy capacity as an experience curve, we find a learning rate of 3%, which reflects a mixture of economies-of-scale and learning-by-doing mechanisms. We also quantify the impact on construction costs of offshore wind power from the recent tightness in the market for turbine manufacturing and installation services: without the demand-supply response inertia at the origin of this tightness we estimate that the learning rate would be 5%.