Perhaps the Civil War Sooner will appear and enlighten us on the true nature of Oklahoma's involvement in the Civil War. Pretty sure he'd know the details of it.
Speak of the devil . . . .
Oklahoma history is not really my subject, but Oklahoma
was Indian Territory at the time. The only federal officials prior to the war were just there to oversee the various Plains Indian tribes that had been relocated there, other than the "Five Civilized Tribes" relocated from what is now called the Southeastern U.S.
The Confederacy gained the loyalty of most of the tribes by granting them representation in the C.S. Congress, and because the Civilized Tribes were slaveholders, like their former white neighbors back in the South. The Cherokees were notably split. The more assimilated, more intermarried-with-whites faction, led by former chief John Ross (7/8 Scotch-Irish, IIRC) stuck with the Union. The less-assimilated, more pure-blood faction, led by principal chief Stand Watie, went with the Confederates. Stand Watie would go on to be the last Confederate general to surrender in 1865.
There were a fair number of small battles fought in Indian Territory. Several of them were inter-tribal Cherokee fights. Indian Territory was more or less secured for the Union with the Battle of Honey Springs, near modern-day Checotah, in July 1863, same month as Gettysburg. There is a re-enactment held there every year. Even though it was the biggest battle fought in the territory, it was still a small affair, with fewer than 10,000 total troops involved. The losing Confederates were the larger force. Both sides contained mostly black and Indian troops.
After the war, the Indians lost more of their land for having sided with the Confederates. The Cherokees got no break for having been split. The western part of Indian Territory was opened to white settlement with a series of land runs beginning in 1889 and ending in 1895. This part of the state was designated Oklahoma Territory, while the eastern part, where the 5 Civilized Tribes and Osages lived, remained Indian Territory. The two sections merged into the State of Oklahoma, the 46th state, in 1907.