Oscar Wilder Underwood was an American lawyer and politician from Alabama, and also a candidate for President of the United States in 1912 and 1924. He was the first formally designated floor leader in the United States Senate, and the only individual to serve as the Democratic leader in both the Senate and the United States House of Representatives.

The Revenue Act of 1913, also known as the Underwood Tariff or the Underwood-Simmons Act, re-established a federal income tax in the United States and substantially lowered tariff rates. The act was sponsored by Representative Oscar Underwood, passed by the 63rd United States Congress, and signed into law by President Woodrow Wilson.