This study examines the peacekeeping activities of the Organization of African Unity (OAU) in resolving the Nigerian crisis. On May 30, 1967 the eastern part of Nigeria, the self-proclaimed Republic of Biafra, tried to secede from Nigeria. This led to a civil war that lasted from July 6, 1967 to January 15, 1970. Biafra’s army was defeated and capitulated. The reference to the history of the OAU peacekeeping experience is relevant, because it can be applied to the settlement of contemporary crises and conflicts in Africa. The author was guided by the principles of historicism, scientific objectivity and reliance on sources. The aim of the article is to clarify the nature and methods of the OAU’s peacekeeping activities, to identify internal and external factors that hindered the achievement of peace, and to assess the effectiveness of the organization’s peacekeeping efforts. The article uses for the first time information and analytical memos of Soviet diplomats found in the Foreign Policy Archive of the Russian Federation (AVP RF) on the OAU’s activities to stop the civil war in Nigeria. The author concludes that the Nigerian crisis was a unique international conflict for the Cold War period. The motives of the external actors were primarily determined by geopolitical aspirations and national interests, rather than bloc solidarity. The author identifies factors that negatively affected the OAU’s potential as a peacemaker: a split among African countries (four of which recognized Biafra’s independence) and competition from Great Britain, which vigorously promoted its own peacekeeping agenda. The OAU’s decisions were not binding on member states; it had no effective mechanism for implementing them, and it had no armed forces of its own that could be used to disengage the warring parties. The OAU succeeded in diplomatically securing overwhelming African support for Nigeria’s territorial integrity, though its mediation efforts failed to achieve peace. The Biafra leadership was not going to capitulate while there was still room for resistance and the federal government was not inclined to question the country’s territorial integrity. The results of the OAU’s peacekeeping can be assessed as positive: it prevented the legitimization of separatist Biafra, which could have had a domino effect with disastrous consequences for the entire African continent.