What is the nickname for the state of Oklahoma?

The Sooner State.

In 1889, the Indian Territory was opened to settlers. Thousands of people lined up on the border and, when the signal was given, they raced into the territory to claim their land. Some people went in early to claim their land. They became known as Sooners. Hence, Oklahoma's nickname today is "The Sooner State".

http://www.50states.com/bio/nickname4.htm

It is the Sooner State.

Oklahoma is the 28th most populous and 20th-largest state. The state's name is derived from the Choctaw words okla and humma, meaning "red people", and is known informally by its nickname, The Sooner State. Formed from Indian Territory on November 16, 1907, Oklahoma was the 46th state to enter the union. Its citizens are known as Oklahomans, and its capital and largest city is Oklahoma City.

https://en.wikipedia.org/wiki/Oklahoma