East Germany joined on 3 October 1990 through reunification with West Germany. In the 1990s, several European countries, now members of the European Union, expressed their willingness to join the Organisation. In 1995, Cyprus applied for membership, but according to the Cypriot government, it was vetoed by Turkey. In 1996, Estonia, Latvia, and Lithuania signed a Joint Declaration expressing willingness to become members of the OECD, and Slovenia also applied for membership that same year. In 2005, Malta applied to join the Organisation. The EU is lobbying for the admission of all EU member states. Romania reaffirmed in 2012 its intention to become a member of the Organisation through the letter addressed by Romanian Prime Minister Victor Ponta to OECD Secretary-General José Ángel Gurría. In September 2012, the government of Bulgaria confirmed it would apply for membership before the OECD Secretariat.
The OECD established a working group headed by ambassador Seiichiro Noboru to work out a plan for the enlargement with non-members. The working group defined four criteria that must be fulfilled: "like-mindedness", "significant player", "mutual benefit" and "global considerations." The working group's recommendations were presented at the OECD Ministerial Council Meeting on 13 May 2004, and on 16 May 2007, the OECD Ministerial Council decided to open accession discussions with Chile, Estonia, Israel, Russia, and Slovenia, and to strengthen cooperation with Brazil, China, India, Indonesia, and South Africa through a process of enhanced engagement. Chile, Slovenia, Israel, and Estonia all became members in 2010. In March 2014, the OECD halted membership talks with Russia in response to its annexation of Crimea.
In 2013, the OECD decided to open membership talks with Colombia and Latvia. In 2015, the Organisation opened talks with Costa Rica and Lithuania. Latvia became a member on 1 July 2016, and Lithuania soon followed on 5 July 2018. Colombia signed the accession agreement on 30 May 2018 and became a member on 28 April 2020. On 15 May 2020, the OECD decided to extend a formal invitation for Costa Rica to join the OECD, which joined as a member on 25 May 2021.
Other countries that have expressed interest in OECD membership are Argentina, Brazil, Croatia, Malaysia and Peru.
In January 2022, the OECD reported that membership talks were underway with Argentina, Brazil, Bulgaria, Croatia, Peru and Romania.
In March 2022, the OECD suspended the participation of Russia and Belarus due to the ongoing Russian invasion of Ukraine.
In June 2022, during the annual OECD Ministerial Council Meeting, the Roadmaps for the Accession to the OECD Convention for Brazil, Bulgaria, Croatia, Peru and Romania were adopted. In March 2024, the Roadmaps for the Accession to the OECD were adopted with Argentina and Indonesia, and in July 2024, also with Thailand.