The Foreign Office (FO) was formed in 1782 and merged with the Commonwealth Office (CO) in 1968 to create the Foreign and Commonwealth Office (FCO). The majority of the material that can be accessed at the Bodleian is the 'published' output from the departments. The exceptions being the Foreign and Colonial Office Confidential Print and papers held in Special Collections.
Unpublished material, the records of the departments are found at the National Archives please see links below.
The Bodleian holds sets of Foreign Office/Colonial Official Confidential Print.
Salient document are also available in digital format for Africa, North America and the Middle East (see links) The print collection covers South America and many other jurisdictions which are not available in digital format.
The Bodleian Library as a legal deposit library has an excellent collection of departmental publications, please search SOLO in the first instance. Below is a list of notable collections available at the Bodleian.