Scholars have long debated the causal impact of international institutions such as the World Trade Organization or the International Monetary Fund. This study investigates Organization of Petroleum Exporting Countries (OPEC), an organization that purports to have significant influence over the market for the world's most important commodity–petroleum. Using four empirical tests, I find that OPEC has little or no impact on its members' production levels. These findings prompt the question of why so many people, including scholars, believe in OPEC's influence over the world's oil supply. The idea of OPEC as a cartel is a “rational myth” that supports the organization's true principal function, which is to generate political benefits for its members. One benefit it generates is international prestige. I test this idea using data on diplomatic representation and find that OPEC membership is associated with increased international recognition by other states. Overall, these findings help one to better understand international regimes and the process of ideational change in world politics.