The municipality was founded in 1759 by the Marquis of Pombal on an area granted as a reward by King Joseph I to his minister for his efforts in rebuilding Lisbon's historical downtown () after the 1755 Lisbon earthquake. A royal charter, dated 7 June 1759, gave jurisdiction over the lands within Oeiras to the King's loyal minister, who became the first Count of Oeiras. A month later, the small village was elevated to the status of town, and then gained municipal status on 13 June 1759. Although Oeiras had a history of earlier settlement, it was during the reign of Joseph I that economic and social development, conditioned by the influence of the King's Minister, who promoted innovation and supported local economic activities, began to flourish.