It started with mud and straw. Around 1697, Jesuits under Padre Miguel Carvalho put up a small chapel beside a stream called the Mocha, in cattle country deep in the Brazilian interior. The settlement that gathered around it took the stream's name: Vila da Mocha. In 1712 King João V granted it the status of *vila*, a formal town. Six years later the Crown split the Captaincy of Piauí off from Maranhão and named Vila da Mocha the new captaincy's capital — a decision made in Lisbon about a place that almost nobody in Lisbon had seen, on the strength of a chapel, a stream and a lot of cattle.

A royal decree is one thing; a functioning government is another. The autonomy granted to Piauí in 1718 translated into very little on the ground for four decades. The first governor, João Pereira Caldas, did not arrive until 1759. Two years after that the settlement was raised to the rank of city and renamed Oeiras, honoring the Count of Oeiras — Sebastião José de Carvalho e Melo, remembered by his later title as the Marquis of Pombal, the minister who rebuilt Lisbon after the 1755 earthquake and who, in 1759, expelled the Jesuits from Portugal and its empire. So the town that began as a Jesuit chapel now carried the name of the man who threw the Jesuits out.

Piauí's break with Portugal was declared here. On 24 January 1823, Oeiras proclaimed the province's adhesion to Brazilian independence, in a movement led by Manuel de Sousa Martins, later Barão da Parnaíba. This was not a ceremony held after the outcome was settled. Independence in the North of Brazil was contested rather than conceded, and Portuguese loyalist forces were still active in the region when the declaration was made. A small inland city with no port and no garrison of consequence had picked a side.

The mother church, the Igreja Matriz de Nossa Senhora da Vitória, anchors the historic center, with the Igreja de Nossa Senhora do Rosário nearby. Crossing the water is the Ponte Grande, described as the first stone bridge built in Piauí — infrastructure from an era when the state's road network essentially was this town and the tracks radiating from it. What makes the ensemble worth the federal listing is not any single monument but the fact that the relationship between the buildings and the ground survived. Colonial capitals that kept their function tend to have their eighteenth-century fabric buried under nineteenth- and twentieth-century ambition. Oeiras never had the ambition, so it kept the fabric.

The year in Oeiras turns on Semana Santa. Holy Week fills the streets with processions — the Procissão do Bom Jesus dos Passos, the Descida da Cruz — and draws pilgrims from across Piauí, giving the town for a few days something close to the crowds it once held as a seat of government. Two patronal feasts punctuate the rest of the calendar: Nossa Senhora da Vitória on 15 August, and Nossa Senhora da Conceição on 8 December. In between it is very hot. On 25 September 2023 the town recorded 42.4°C, one of the higher readings taken anywhere in Brazil that year. The 2022 census counted 38,161 residents, which makes Oeiras a small city that used to run a state.