The current state flag of Oklahoma, designed by Louise Fluke, was adopted in 1925 (Oklahoma had 13 previous flags). The blue field signifies devotion, the shield is a symbol of defensive or protective warfare, but always surmounted by the olive branch and peacepipe which betoken the love of peace by a united people.

Description of the flag from official statutes:

"A sky blue field with a circular rawhide shield of an American Indian Warrior, decorated with six painted crosses* on the face thereof, the lower half of the shield to be fringed with seven pendant eagle feathers and superimposed upon the face of the shield a calumet or peace pipe, crossed at right angles by an olive branch ... and underneath said shield or design in white letters shall be placed the word "Oklahoma", ... hereby adopted as the official flag and banner of the State of Oklahoma."

*white crosses are the Indian symbol for stars.

Source:

Oklahoma state icons: Oklahoma's official website

Links:

OKLAHOMA'S HISTORY: OK.gov
All State Flags - National Flag