The Statute of the International Court of Justice describes customary international law as „a general practice accepted as law”. It is generally agreed that the existence of a rule of customary international law requires the presence of two elements, namely State practice (usus) and a belief that such practice is required, prohibited or allowed, depending on the nature of the rule, as a matter of law (opinio juris sive necessitatis). This paper aims to reseach the impact that an International Organization has on both: confirming and shaping a rule of customary international law.