Oeiras is a municipality in the Microregion of Picos of the Brazilian State of Piauí. According to the Instituto Brasileiro de Geografia e Estatística (Brazilian Institute of Geography and Statistics), its population was 34,538 inhabitants in 2003. The municipality covers an area of 2720 km².
Oeiras grew up around a church founded in 1695 dedicated to Our Lady of the Victory. Oeiras was elevated to the level of town in 1712, then became the capital of Piauí in 1759. In 1761 it became a city. The capital was transferred to Teresina in 1851.