The law establishing the framework for intermunicipal communities and metropolitan areas was approved by the Assembly of the Republic (Assembleia da República) on 27 August 2008. On 25 November 2008, the Associação de Municípios do Oeste (Association of Municipalities of the West), by the approval of the municipal assemblies (assembleias municipais) of each of its constituent municipalities, converted itself into the Comunidade Intermunicipal do Oeste. The law formally establishing the names, borders, and duties of the intermunicipal communities and metropolitan areas was approved by the Assembly of the Republic on 12 September 2013.
OesteCIM is the successor to Associação de Municípios do Oeste (Association of Municipalities of the West) and Comunidade Urbana do Oeste (Urban Community of the West; abbreviated ComUrb Oeste), which had been instituted on 29 March 2004.
OesteCIM is coterminous with the statistical NUTS III subregion Oeste within the NUTS IICentro Region. It is a region famous for its fruits and vegetables production, namely the famous pêra rocha variety.
Headquarters of Comunidade Intermunicipal do Oeste, originally built for Associação de Municípios do Oeste. Rio Maior, whose name appeared on the façade when the photo was taken, did not join the successor entities.
Comunidade Intermunicipal do Oeste is made up of twelve municipalities:
^"Constituição de associação". Diário da República — III Série N.º 99 — 27 de Abril de 2004 (in Portuguese). Comunidade Urbana do Oeste, via Imprensa Nacional-Casa da Moeda, S. A. 27 April 2004. pp. 9182–(8)–9182–(15). Retrieved 29 April 2014.