The United States acquired the area of Oklahoma from France as part of the Louisiana Purchase in 1803. Part of present-day Oklahoma, which had been included in Arkansas Territory, was ceded to Spain by conclusion of a treaty in 1819; the ceded area was reacquired as part of the annexation of Texas in 1845. The area of Oklahoma was part of unorganized territory designated as Indian Country or Indian Territory on June 20, 1834, although this conflicted with territory already included within Missouri Territory. Oklahoma Territory was organized on May 2, 1890, from the western part of Indian Territory and the Public Land Strip (the panhandle, which was sold to the United States by Texas), and resulted in the territory being in two pieces. The territory was enlarged with the addition of the Cherokee Outlet, which joined Oklahoma Territory into a single area. The Supreme Court affirmed the claim of Oklahoma to the Greer County area in southwest Oklahoma in 1896. Oklahoma Territory and the remaining Indian Territory were combined, and Oklahoma was admitted to the Union on November 16, 1907, as the 46th state, with generally the same boundary as the present state. A boundary dispute with Texas was settled in 1930, deleting a narrow strip of territory from Oklahoma.

Census data are available for Oklahoma beginning with the 1890 census. The 1890 and 1900 census populations reported for Oklahoma include the population for Indian Territory. The population of Oklahoma Territory as legally established was 398,331 in 1900 and 78,475 in 1890. The population of Indian Territory as legally established was 392,060 in 1900 and 180,182 in 1890. The Census Bureau conducted a special census of the Oklahoma and Indian territories on July 7, 1907. The population of the entire area was 1,414,177.

Data for the legally established state of Oklahoma are available beginning with the 1910 census.