As long as we're at it, "Sooners" were predominantly white folk as well, but as some would point out there is no way of knowing for sure that ALL the Sooners were white.
There were many freed blacks in the "old west" of 1907, and possibly immigrants from Mexico or other places, but its probably safe to say that the SOONERS were PREDOMINANTLY, but NOT EXCLUSIVELY, white people.
ALMOST as white as the "Cornhuskers" (Defined as the population of Nebraska), but probably whiter than the general population of Cowboys.
For those who pick nits over that sort of thing...