The Organization of American States（OAS）is the most important regional organization，founded and dominated by the US. As the most important multilateral policy tool of the US towards Latin America，the OAS’s roles and influences in the Western Hemisphere system are closely related to the US’s foreign strategies，its position of strength in international system and its dominant power in Latin America. Since its founding in 1948 to the 1960s，the US played an overwhelmingly dominant role in the organization. However，in the late 1960s，with the power of the US declined，its adjustment to foreign policies，and more independent foreign policies formed in Latin American countries，the OAS stepped into its low tide period since then. After the Cold War，pushed by globalization and adjustment of US foreign policy，the OAS was revitalized. But in the 21st century，the OAS again went downhill with the decline in the US’s power and rise of Latin American left wings. Due to more and more internal divergences and challenges from emerging sub-regional organizations，the OAS has a dim future，but it still plays irreplaceable roles in regional governance of low politics. Therefore，the US and Latin America countries will continue to keep it forward.