An optimal currency area (OCA) is a currency region in which the benefits of adopting a single currency, such as economies of scale and macroeconomic policy effectiveness, outweigh any cost associated with sacrificing economic sovereignty. The concept was first introduced by Nobel Prize-winning economist Robert Mundell in the early 1960s. He sought to answer the question of why some countries have opted to keep their own currency and monetary policy, while others have chosen to adopt one regional currency and a common monetary policy.

Mundell’s theory of an OCA is based on two key criteria:  economic integration and economic similarity. Economic integration refers to the level of trade and investment between countries within a region. Economic similarity refers to similar economic structures and the degree of macroeconomic policy coordination among countries. According to Mundell’s criteria, the more integrated and economically similar the countries are, the more likely it is for them to form an OCA and for the monetary union to be successful.

The euro was meant to be an example of an OCA put into practice. The euro was adopted in 1999 and currently functions as currency for 19 European countries. To create an OCA, the European countries had to meet the criteria of economic integration and economic similarity. Initially, this was the case, but the Greek debt crisis and other economic difficulties have threatened the stability of the euro. This has highlighted the issues with having a monetary union on such a large scale, including the lack of fiscal coordination and the struggle to define who has responsibility for the problem countries.

For a monetary union to be successful, countries must work together to fix underlying economic issues. This involves not only coordination over fiscal and monetary policy, but also greater political integration in some industries, such as banking. When countries make decisions that defy the collective interests of the region, further currency shocks can result that could destabilize the region's financial system.

In summary, an optimal currency area is a region where its countries have met the criteria of economic integration and economic similarity, allowing for the adoption of a single unified currency. The euro was meant to be an example of an OCA put into practice, but the Greek debt crisis has highlighted the difficulty of having a unified monetary policy across 19 different countries with no fiscal coordination. For a region to adopt an OCA successfully, there needs to be a closer relationship between countries in terms of fiscal and monetary policy, as well as greater political integration in certain sectors of the economy.