Wordorigins.org

Oklahoma is from the Choctaw okla, people, and humma or homma, meaning red, so it can be construed to mean land of the Indian. When the Indian territory was established in what is now the state, Allen Wright, chief of the Choctaws, suggested this as the name. When it was formally organized as a US territory in 1890, the Indian name for the land was adopted as its formal name. Oklahoma was admitted to the union in 1907.

(Sources: Illustrated Dictionary of Place Names; Webster’s Third New International Dictionary)