OECS: History

1966 The West Indies Associated States Council of Ministers (WISA) is formed.
The Eastern Caribbean Common Market (ECCM) is formed. 1968
1981 As the islands gain their independence from Britain it becomes evident that there is need for a more formal arrangement to assist with their development efforts. As a result, the OECS is established via the Treaty of Basseterre. The West Indies Associated States Council of Minsters (WISA) Secretariat becomes the central secretariat of the OECS and the Eastern Caribbean Common Market (ECCM), the Economic Affairs Secretariat.
British Virgin Islands join OECS. 1984
1995 Anguilla joins OECS.
Due to the restructuring of the organization the Economic Affairs Secretariat iss merged into and becomes a Division of the OECS Secretariat in St Lucia. 1997
2010 Revised Treaty of Basseterre is signed, which establishes the OECS as an Economic Union, making possible the creation of a single financial and economic space within which goods, people, and capital move freely, monetary and fiscal policies are harmonized and countries continue to adopt a common approach to trade, health, education, and environment as well as to the development of such critical sectors as agriculture, tourism, and energy.
Martinique joins the OECS. 2015

Sources

Caribbean Community Secretariat: Organisation of Eastern Caribbean States (OECS)

Glossary