Most medium sized cities in the midwest are experiencing a lot of growth. It doesn't just extend into Oklahoma. Omaha and Des Moines are growing as well.
The problem, however, is that most of this growth is fueled by people moving from rural areas of those states into the cities because there are no jobs or opportunities in rural areas... so it's not really a positive net growth, just the centralization of the state population. This is especially true on the plains.
I thought the plains states in general were losing population or growing at a much slower rate in general in relalation to the rest of the country.
The only state that actually has negative population growth is North Dakota. All other states have positive growth but you are correct in saying the plains are growing significantly slower in comparison to the national average. Also, as snowguy pointed out, the rural plains and small towns are bleeding population into the big metro areas so most Midwestern cities maintain a healthy growth rate.
As for Oklahoma City being an essential American city, I think it already qualifies in that category. I'd say that all of the top 50 most populated metro areas are vital in some way to the American economy. I don't think that Oklahoma City will be seeing any great leaps in significance due to a relative small amount of population growth though. There are a lot of other metro areas experiencing much greater growth and most of the top 50 metro areas are still going strong.
