It originated as the Organization for European Economy Co-operation (OEEC), to help administer the Marshall Plan for the re-construction of Europe. But latter its membership was extended to non-European states, and thus it was given its current name.
There are twenty-four full members, nearly all in the 'high-income' group (as defined by the World Bank), it is known as the "rich man's club."
Members are: Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Japan, Luxembourg, Netherlands, New Zealand, Norway, Portugal, spain, Sweden, Switzerland, Turkey, UK and US.