This paper contributes to the research project on the OSCE Legal Framework led by the Max Planck Institute for Comparative Public Law and International Law. It deals with the OSCE’s international responsibility by distinguishing three different but correlated phenomena: (1) its capacity to develop a legal system; (2) its possession of legal personality, whether international or domestic, does not compromise the existence of the institution as such; and (3) how its international responsibility is dependent upon the fact that the OSCE is either considered as founded by an instrument of international law or by an act of creation not based on international law. The introduction sets the scene describing the work of the International Law Commission. Section 2 further discusses the interaction between legal personality and the development of an ‘original’ or ‘derivative’ legal system. Section 3 then discusses four possibilities: (1) the OSCE possesses a derivative legal system and does not have international legal personality; (2) the OSCE possesses a derivative legal system and does have international legal personality; (3) the OSCE possesses an original legal system and does not have international legal personality; and (4) the OSCE possesses an original legal system and does have international legal personality. This paper does not define once and for all what the OSCE is and how its legal responsibility is to be assessed, but instead discusses the potential consequences that different legal constructions would have on its responsibility.The paper contributes to the law of international organizations analyzing how their international responsibility is affected by the adoption of one or another concept of legal system.