Oklahoma is the 20th-largest state in the United States, covering an area of 69,898 square miles, with 68,667 square miles of land and 1,231 square miles of water. It is one of six states on the Frontier Strip, and lies partly in the Great Plains near the geographical center of the 48 contiguous states. It is bounded on the east by Arkansas and Missouri, on the north by Kansas, on the northwest by Colorado, on the far west by New Mexico, and on the south and near-west by Texas.

Oklahoma is situated between the Great Plains and the Ozark Plateau in the Gulf of Mexico watershed, generally sloping from the high plains of its western boundary to the low wetlands of its southeastern boundary. Its highest and lowest points follow this trend, with its highest peak, Black Mesa, at 4,973 feet above sea level, situated near its far northwest corner in the Oklahoma Panhandle. The state's lowest point is on the Little River near its far southeastern boundary in or near the town of Idabel, OK, which dips to 289 feet above sea level.