According to the 1990 Census, Oklahoma had more American Indians—252,420—than any other state. But in 1998 California surpassed the population. According to the census in 2000, Oklahoma has an Indian population of 281,000, second only to California. Oklahoma was also home to some of the nation's largest Indian reservations, including those of the Creek, Cherokee, Chickasaw, and Choctaw Indians.

Black slaves came to Oklahoma, then known as Indian Territory with their Indian masters after Congress forced the resettlement of Indians from the southeast to lands west of the Mississippi River in 1830. By the time of the Civil War, there were 7,000 free Negroes in Oklahoma. After the depression of the 1930s,were concentrated mostlyin Oklahoma City and Tulsa. In 2000, the black population of 260,968 was smaller than the American Indian population.

Mexicans who came to Oklahoma during the 19th century as laborers on railroads and ranches, and in coal mines, today live mostly in Oklahoma City, Tulsa, and Lawton. In 2000, Oklahomans who were classified as Hispanics or Latinos numbered 179,304 and represented 5.2% of the state's total population. Of this total, 132,813 were Mexican.

Italians, Czechs, Germans, Poles, Britons, Irish, and other people of European origin also came to Oklahoma during the 19th century. Persons claiming at least one specific ancestry group in 2000 included English, 291,553; German, 435,245; and Irish, 354,802. In 2000, the Asian population numbered 46,767 and there were 2,372 Pacific Islanders.