The aim of the diploma thesis was to analyze the success of the Organization for Security and Co-operation in Europe (OSCE) during the mandate of its Mission in Croatia. By analyzing the OSCE's official working reports, we have shown that, through its eleven-year mission mandate, significant results have been achieved. For a better understanding, we have presented the structure of the OSCE and the characteristics of its activities, in order to distinguishing it from other organizations operating in the area of European collective security. We paid special attention to the achievements of the Mission in the field of human rights and the rights of national minorities, since interventions in this area were extremely important in the post-war period. The thesis has shown how the Mission influenced the democratic development, the stability of the state and the region, and further positioning of Croatia in the Euroatlantic area. Measures implemented by OSCE, by which Croatia was strengthened in the political, economic and social field, facilitated the later fulfillment of the conditions that stood in the way of one of the main foreign policy goals of Croatia's- accession to the European Union. Diploma thesis has thus provided insight into the utility of the Organization for European Security and Cooperation as a unique example of a platform for inter-state cooperation through peaceful means of action.