Electricity generated by wind turbines (WT) is a pillar of the transition to renewable energy [1]. In order toeconomically utilize WTs, operating and maintenance costs, which account for 25% of total electricitygeneration costs in onshore WTs, are a focus of cost reduction activities [2]. A prescriptive maintenanceapproach can support in achieving this goal. Prescriptive maintenance is a maintenance approach, whereasset condition data is collected and analyzed to recommend specific actions to prevent breakdowns andreduce downtimes. However, the processing and analysis of data is quite complex. Especially unstructureddata (such as comments of service technicians in free text fields) is often left unused, as companies, mostlySMEs lack the capacity to carry out these analyses. In this work we propose an approach to utilize theinformation from service reports, maintenance reports as well as status records from SCADA systems forthe development of a prescriptive maintenance approach to onshore WTs. To achieve this, an ontology wasutilized in this approach to codify implicit knowledge of service technicians and aid in making unstructureddata usable for further analysis. The ontology was used to link historical service and maintenance reportswith status codes, thus enabling automated analysis. In interviews with WT topic experts and through furtherresearch, damage mechanisms and corresponding maintenance measures were identified and a measurecatalogue was developed to support service and maintenance activities. The recognition of the root cause ofproblems allows for a prescriptive maintenance approach that recommends targeted actions to reducedowntimes and optimize maintenance activities, it also allows to effectively control the outcome ofmaintenance activities and optimize their execution.