Independent oversight institutions are critical components of the accountability landscape in modern democracies. This paper presents an outline for indexing the accountability powers of these watchdogs. This watchdog accountability index is an empirical tool to assess the key accountability powers of accountability forums that operate in a democratic constitutional context. The aim is to provide a richer evidence base to assess evolving external accountability arrangements and their effectiveness. We apply the accountability index to assess the strength of two main watchdog institutions in the EU, the European Ombudsman (EO) and the European Court of Auditors (ECA). Data are collected by means a study of secondary sources and analysis of annual reports and other relevant documents. A first comparison of the accountability index assessing the power of the two institutions shows clear differences between the EO and the ECA.