An oil well is a drillhole boring in Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas may be termed a gas well. Wells are created by drilling down into an oil or gas reserve and if necessary equipped with extraction devices such as pumpjacks. Creating the wells can be an expensive process, costing at least hundreds of thousands of dollars, and costing much more when in difficult-to-access locations, e.g., offshore. The process of modern drilling for wells first started in the 19th century but was made more efficient with advances to oil drilling rigs and technology during the 20th century.

Wells are frequently sold or exchanged between different oil and gas companies as an asset – in large part because during a drop in the price of oil and gas, a well may be unproductive, but if prices rise, even low-production wells may be economically valuable. Moreover, new methods, such as hydraulic fracturing (a process of injecting gas or liquid to force more oil or natural gas production) have made some wells viable. However, peak oil and climate policy surrounding fossil fuels have made fewer of these wells and costly techniques viable.

However, neglected or poorly maintained wellheads present environmental issues: they may leak methane or other toxic substances into local air, water and soil systems. This pollution often becomes worse when wells are abandoned or orphaned – i.e., where a well is no longer economically viable, so are no longer maintained by their (former) owners. A 2020 estimate by Reuters suggested that there were at least 29 million abandoned wells internationally, creating a significant source of greenhouse gas emissions worsening climate change.

The earliest known oil wells were drilled in China in 347 CE. These wells had depths of up to about 240 metres (790 ft) and were drilled using bits attached to bamboo poles. The oil was burned to evaporate brine producing salt. By the 10th century, extensive bamboo pipelines connected oil wells with salt springs. The ancient records of China and Japan are said to contain many allusions to the use of natural gas for lighting and heating. Petroleum was known as burning water in Japan in the 7th century.

According to Kasem Ajram, petroleum was distilled by the Persian alchemist Muhammad ibn Zakarīya Rāzi (Rhazes) in the 9th century, producing chemicals such as kerosene in the alembic (al-ambiq), and which was mainly used for kerosene lamps. Arab and Persian chemists also distilled crude oil in order to produce flammable products for military purposes. Through Islamic Spain, distillation became available in Western Europe by the 12th century.

Some sources claim that from the 9th century, oil fields were exploited in the area around modern Baku, Azerbaijan, to produce naphtha for the petroleum industry. These places were described by Marco Polo in the 13th century, who described the output of those oil wells as hundreds of shiploads. When Marco Polo in 1264 visited Baku, on the shores of the Caspian Sea, he saw oil being collected from seeps. He wrote that "on the confines toward Geirgine there is a fountain from which oil springs in great abundance, in as much as a hundred shiploads might be taken from it at one time."

In 1846, Baku (settlement Bibi-Heybat) the first ever well was drilled with percussion tools to a depth of 21 metres (69 ft) for oil exploration. In 1846–1848, the first modern oil wells were drilled on the Absheron Peninsula north-east of Baku, by Russian engineer Vasily Semyonov applying the ideas of Nikolay Voskoboynikov.

Ignacy Łukasiewicz, a Polish pharmacist and petroleum industry pioneer drilled one of the world's first modern oil wells in 1854 in Polish village Bóbrka, Krosno County, and in 1856 built one of the world's first oil refineries.