O'Neill House Office Building, Congressional office building in Southwest Federal Center, United States.

The O'Neill House Office Building is a modern office block at 200 C Street Southwest in the Southwest Federal Center, at the foot of Capitol Hill. It stands between the Hubert H. Humphrey Building and the Ford House Office Building, next to the Center Leg Freeway of Interstate 395. It holds offices of the House of Representatives and of the Department of Health and Human Services, with about 2,000 staffers.

Congress approved plans in 1960 for Federal Office Building No. 8, dedicated by the General Services Administration in 1965 at a cost of $19.5 million to house Food and Drug Administration headquarters and animal laboratories for up to 20,000 animals. A $130 million renovation began in 2008, earning LEED gold. The House voted in 2012 to name it for Speaker Tip O'Neill, and it opened for occupancy in 2014.

The building is named for Thomas P. "Tip" O'Neill Jr. of Massachusetts, Speaker of the House from 1977 to 1987; the House voted for the name in 2012 on a suggestion by then minority leader Nancy Pelosi. The name was applied after the 2017 law transferring the building to the Architect of the Capitol.

The entrance is at 200 C Street Southwest, between the Hubert H. Humphrey Building and the Ford House Office Building; the Center Leg Freeway of Interstate 395 runs along the side facing the Rayburn House Office Building. Access is secure: visitors need an appointment and an escort. The Southwest Federal Center sits at the foot of Capitol Hill.

The building was designed to hold as many as 20,000 animals when it served as Federal Office Building No. 8 for the Food and Drug Administration, alongside 1,200 workers. Its 2008 renovation cost $130 million and earned a gold LEED rating for energy- and water-saving features. The House voted in 2012 to name it after Tip O'Neill on a suggestion by Nancy Pelosi.