Oklahoma really doesn't fit in with Iowa or the Dakotas. It's just the South on Midwestern soil: mostly Baptist, very conservative, lots of guns and sweet tea and pickup trucks. It's flat, sure- but so is Arkansas.
You may be better off splitting the Midwest into a northern and a southern portion. This is most apparent in politics (the northern Midwest voted for Obama in the last two elections, which is unimaginable in Kansas or Oklahoma), but is more far-reaching than that.
Also, Cleveland, Detroit, Buffalo, and to an extent Pittsburgh (which however has undergone a resurgence in the last couple of decades) are all Rust Belt. That's definitely a cultural grouping: post-industrial cities and their hinterlands, mostly populated by Eastern Europeans and some Italians and Irish, with bad ghettos, that were much more prosperous and populated half a century ago.
You may be better off splitting the Midwest into a northern and a southern portion. This is most apparent in politics (the northern Midwest voted for Obama in the last two elections, which is unimaginable in Kansas or Oklahoma), but is more far-reaching than that.
Also, Cleveland, Detroit, Buffalo, and to an extent Pittsburgh (which however has undergone a resurgence in the last couple of decades) are all Rust Belt. That's definitely a cultural grouping: post-industrial cities and their hinterlands, mostly populated by Eastern Europeans and some Italians and Irish, with bad ghettos, that were much more prosperous and populated half a century ago.