In 1788, the Hesse District was established within Upper Canada covering the territory of what is today southwestern Ontario. Four years later it became the Western District with the establishment of Norfolk County which included the territory of present-day Oxford County.
In 1798, these lands were included into a new London District. The Brock District, containing the Oxford County territory, was then split off from the London District in 1840, after Upper Canada had been replaced by the Canada West portion of the Province of Canada governance.
In 1855, Norwich Township in the county's southeast was divided into North and South townships to accommodate significant population levels in that area. Except for this adjustment, the township boundaries within the county remained intact until the late 20th century.
In January 1975, major revisions to Oxford County's structure took effect when the townships were amalgamated to the current five. Three urban municipalities, Ingersoll, Tillsonburg and Woodstock, also remained. Today's county boundaries also show slight adjustments to include the urban areas of Tavistock in the north, and of Tillsonburg in the south.